Howdy

Lift pump pulls fuel from your tank and "lifts" it to the IP=injector pump(main).
The IP is located in the intake valley and would be where all your lines come from. If this van has the original pump and injectors then it might be due for a rebuild of both. I have never had a 6.2 fail likes your does because of worn pump or injectors. Performance normally gets so bad that they get rebuilt before they completely fail. A common issue is not starting hot or hard starting all the time. Sounds like yours wants to start and then dies.
My 1984 van only has the square filter on the back of the engine. I would suspect that if you have another type of round filter that it was an after thought by someone. I don't think they ever came with both a square filter and a round filter.
I think the 6.2s that came with two spin on filters had one located in the intake valley rear and one on the fender wall. Not sure how that would locate on your RV.
Your van SHOULD have a mechanical lift pump that runs and looks just like the fuel pump on older gas job GM motors: mounted on the front of the engine block low on the passenger side. If there is an electric inline pump it was added either as way of priming the system for any number of reasons or as a "simpler" fix to the mechanical lift pump failing. Some people put them inline with the mechanical pump. Unless you have an oil pressure shut off for the electric pump, it is a bad idea.
IMHO, the lighting problem sounds like a ground or fixture problem (if the crab is good). Could possibly be the wiring from the crab to the flat connector under the column. maybe something is shorted in there from a PO doing some work in the column.
Hope this helps!
Good luck.
Sounds like your fuel system has been extensively "modified" If it was mine I would start by simplifying that. Get back to one good filter(square) and one good mechanical pump with unspliced lines.
As to the voltage issue, I would first put a meter on it before you get to concerned. The factory gauges are not the best.