It’s been 2 yrs since, I didn’t take any phots or notes, only memories of working in cramped space above my head while lying on gravel in alley.
If it was unusual I think I might remember but not so I believe the connector was just a standard length (6 to 10 inches ?) of flexible hose with standard hose clamp that is tightened with screwdriver or small socket driver. I may have, likely, cut the connector hose with a knife for quick removal and possibly replaced the hose with a length bought at hardware store.
I don’t recall any detail of the upper tank. I don’t believe I could see it from below. I would guess now that there was only the connector hose extending through the floor or possibly just the “connector nipple” exposed that I could connect the lower tank to. If as you suspect, the leak is from the gravity fill hose, that is a possibility too. Due to the layout of the fill port on the door post, the fill hose must connect to the tank (below the seat) at or below floor level, thus at bottom of the tank.
Regardless of exactly where the leak is I suspect you will have to either remove the seat supports as someone explained or the lower tank, both significant effort.
I do remember the re-installation included re-attaching individual electrical wires to each of the three or four sensors for 1/4, 1/2, 3/4 full on the fresh and black or grey tanks I had to remove. These sensors didn’t, before or after, work well. I’ve since seen a different type of sensor system that seems like an improvement. It is a strip of sensor tape you glue to the outside of the tank and somehow connects to a monitor panel located conveniently inside the RV. It’s been awhile since I’ve seen / thought about that as well but it’s something I’d look up and probably use instead of the original system.
The tank removal / replace process didn’t require any special tools except maybe a socket set extension. Making a ramp about 20 to 24 inches high to raise the front of the van and maybe 8 inches for rear was my biggest scrounging need. The fresh tank shape is such that this clearance was needed to lower it for removal. You may not need to fully remove it and thus be OK with a bit lower ramp but you still need space for you body to move around the van.
Seems like a major effort to diagnose and hopefully resolve the problem but ...
Good Luck !