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Old 02-14-2020, 08:42 PM   #21
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I agree with Dave regarding dumping frequency, from my perspective it is indeed the largest differentiator with threshold being every 3-4 days. So, this threshold, including accessibility to dumping stations, could be the key decision driver for cassette or black tank. We are below these 3-4 days, so cassette works for us. In Dave’s usage I would also pick black tank if there is available mounting space.

Regarding single seal my experience is different, it could be the result of different experiences, Pete’s one season with one cassette, mine multiyear with Thetford 402C. Every time I remove the cassette the cassette seal is tested simply be existence of any seepage, with the cassette in the vertical position this seal needs to take a few inches of water pressure. In addition, we tend to keep some water in the bowl which doubles as a seal. Boiling down to the key point - our cassette doesn’t generate any inside odors but it is vented to outside per design. Odor could be generated by a contaminated main housing not a seal failure.
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Old 02-14-2020, 11:27 PM   #22
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For the record I have and 18 gallon black tank and a 28 gallon grey tank. If you have a cassette you still have to frequent dumping stations for the grey tank unless you are like the German fellow in Anchorage, Alaska I observed emptying his grey tank one bucket at a time with an attached garden hose. Or worse the guy who just empties on the ground the campsite just before you arrive.

As for combined tanks there are four things in a Class B that hinder it:

1. The black tank rules from the location of the toilet. You are limiting the possibilities of design placement to optimize the tank capacity.

2. with the underside small frame of Class B's it could be more advantages to split up the grey and black tank to take advantage of more space. You probably cannot find contiguous space as great as two separate spaces and of course item 1 also would come into play.

3. Though tanks are vented, there is a frequency of the grey tank traps to dry out in three places, the galley sink, the bathroom sink and the bathroom floor drain. Dry, hot climates and up and down mountain driving exacerbate this and of course there is no prevention of backing up in the floor drain. Grey tank smells are foul but black tanks are much, much worse. The black tank odor trap is the toilet which one can manage easily with the water trap and visual inspection.

4. Emptying the black tank first and the grey second flushes out the macerator and hose of cling ons.In fact, since I take on more water at usually the same time, I flush my fresh tank into both black and grey for more flowage.
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Old 02-14-2020, 11:45 PM   #23
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Originally Posted by Boxster1971 View Post
Has anyone considered a single combined black and grey tank like some of the big Class-As have? Seems like a single tank on a Class-B would give maximum capability in limited space. Of course a single tank would not allow separate dumping of grey water if that is your useage model. I've never dumped my grey tanks anywhere but at a dump station.
I had a combined black/gray tank on my first van, a first-generation Airstream Interstate. It was wonderful. As you suggest, it is pretty much better in every way--more liquid to dilute the black contents, maximum use of available space without wastage if your tanks aren't "balanced" to your usage, the convenience of only a single dump, lower cost and lower weight for a given tank size. It is an excellent option--the only reason they aren't common is hidebound habit on the part of the RV industry.

If you need multiple tanks for configuration reasons, just connect them together. If you actually have issues with traps drying out (which I have literally never experienced), there are dry-traps available. About the only real disadvantage is that the shower drain is low which can be problematic in some designs. This is routinely solved by putting a tub drain closure (or a simple plug) into the drain for use when traveling. The step-to-close/open ones work well and is what Airstream used. You can also solve the issue by putting a back-flow preventer device in the drain.

I didn't have the option for a single tank in my current rig, but if I did, I would have taken it in a heartbeat.
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Old 02-15-2020, 12:47 AM   #24
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Originally Posted by avanti View Post
I had a combined black/gray tank on my first van, a first-generation Airstream Interstate. It was wonderful. As you suggest, it is pretty much better in every way--more liquid to dilute the black contents, maximum use of available space without wastage if your tanks aren't "balanced" to your usage, the convenience of only a single dump, lower cost and lower weight for a given tank size. It is an excellent option--the only reason they aren't common is hidebound habit on the part of the RV industry.

If you need multiple tanks for configuration reasons, just connect them together. If you actually have issues with traps drying out (which I have literally never experienced), there are dry-traps available. About the only real disadvantage is that the shower drain is low which can be problematic in some designs. This is routinely solved by putting a tub drain closure (or a simple plug) into the drain for use when traveling. The step-to-close/open ones work well and is what Airstream used. You can also solve the issue by putting a back-flow preventer device in the drain.

I didn't have the option for a single tank in my current rig, but if I did, I would have taken it in a heartbeat.
Actually, I'd go as far as to say MORE people are moving away from preferring black tanks. This is one of many RV trends that are rapidly changing over to European preferences. I've dealt with black tanks before and I will never have one.

In any case, it's not a contest, so if you prefer a black tank, more power to you, but don't assume that everyone (or even most people) agrees. I'm just letting you know that black tanks aren't as popular as they use to be, especially as more convenient, more sanitary options become more prevalent.
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Old 02-15-2020, 12:55 AM   #25
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Lower cost, lower weight, convenience of a single dump are true, and Airstream once had a combined tank, why is it so "hidebound" of the RV Industry to not do it? Maybe they know something from experience in selling more B's because if it is so great competitive pressures would make it so.

I have the Hepv0 valves (dry traps) you speak of instead of water traps on my sinks but the shower floor drain for some reason they did not install one. Why, I can't tell you. I haven't asked. But I suspect it is for some practical reason or experience since ARV spares no expense if they deem something is better.
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