Getbuck,

A couple of thoughts;

The battery under the curb side. Is it mounted to the frame rail? I've got a Turtle Top converted E350 that has one there. It charges off of the alternator, but is wired with a relay such that it isn't a "starting" battery. It powers all of the accessories like the rear heat/air, lights, etc., and on mine, the electric curbside door opener and my backup alarm. I can tell when that battery is getting weak when the bus is first started, the door won't operate and the backup alarm sounds weak. Then they start working once that battery has built up some juice. It has a relay that sends current to the battery once the bus is running. I think it is activated by alternator output. The point is, if that battery is bad, it is going to put a lot of strain on your alternator. It is a pain to replace, but do it anyway.

I've got a '96 E350 that has a second, smaller "accessory" battery under the hood. It is of similar circuitry design in that isn't in the starting circuit. It powers the trailer circuits. We were having issues with our trailer lighting. That battery was internally shorted and killing everything. About 3 years later, we had similar problems, and since the only lighting that we needed were the running lights, we bypassed that battery rather than replacing it again ($150 - 175 for an Interstate). No problems since. IF we were running a trailer that utilized the trailer accessory power circuit for lighting, sound system, etc. then keeping that battery functional would be important.

The Buss Hi-Amp unit that you photographed is a circuit breaker. What is wired into it, I don't know. I've got one like that on my F350 for my liftgate, so I would assume it is for your wheelchair lift. But, it could be for all of the stuff; lighting, heat/air, wheelchair lift, etc. in back. It could be part of that rear battery circuit as well.

The 20A fuse coming off of the rear circuit relay looks more like a power take-off for something else...perhaps for the high idle controller or something similar. Hard to say with out tracing it.

I DON'T like the ground wires that appear to be sitting right on top of that relay's positive terminal post...that just looks like short waiting to happen.

Do you have ANY power on that rear terminal block about the bus door?

Bruce