The Battery Tender brand is often recommended on forums as being good. I have two of the BatteryMINDer brand myself.
If you have a newer Roadtrek I don't think you need a battery tender though. The Tripp-Lite inverter/charger for the house battery coupled with the under-the-hood Sure Power Battery Separator for the chassis battery should keep all of your batteries nicely maintained.
The '04 Roadtrek that I owned had a basic Hehr Power Systems isolator under-the-hood - not
a Sure Power Battery Separator - so it did not allow the chassis battery to be maintained by the Tripp-Lite inverter/charger. I used a BatteryMINDer to keep the Roadtrek (Chevy) chassis battery charged because there was a parasitic drain that would discharge the chassis battery in two or three weeks if I didn't use the van. Keyless entry systems, compasses, in-dash radios etc. all continually consume a little bit of current.
With a newer Roadtrek (and maybe other Class B Brands) just plugging into grid power should keep all batteries charged if everything is wired up right and the parts are all working.
Test it with a volt meter or multimeter. If the van has been plugged in for a day or more check the voltage at the house battery and the chassis battery. They should be the same and at at least 13.1 volts to 13.6 volts on the high side. Some "smart" chargers periodically put out a boost charge so don't be to alarmed if you see up to 14.4 volts for 15 minutes or so. If the house and chassis battery voltages are not the same then your system is not setup up to maintain all batteries when plugged in.
You don't want to maintain batteries at higher than 13.6 volts though. Even that voltage is on the high side in my opinion. Older chargers in vans constantly supplied 13.6 volts or 13.8 volts and that is not good for long term maintenance. My current van is plugged in all the time at home. All batteries are joined together by a manual selector switch in my situation. I have a digital display on my inverter that I turn on to check the voltage whenever I go into the van just to keep an eye on things.
For anyone with a "smart" charger in their van and simple isolator under the hood you can manually join the two banks using a jumper wire to allow the "smart" charger to maintain the house and chassis battery banks. See this: http://www.classbforum.com/phpBB2/vi...2&t=453&p=1038
I don't recommend it simply because it easy to forget the jumper wire under the hood. Put a reminder tag on the van steering wheel to remind you to always remove the jumper before starting the van and driving off. The jumper is handy to have as you could use your house batteries and/or generator to give your van a jump start if the chassis battery went dead while camping.
Every RVer should have a way to measure battery voltage. If you are plugged in for a day or more and see 13.1 or 13.2 volts at all batteries for newer Class B's then that is ideal. You should at least see that voltage at the house batteries for maintenance when plugged in.