newbie battery and electrical question

I would got to FORD and get a body builders handbook for your E series. Ford produces these to prevent up fitters, especially from the RV industry from doing things that will void the warranty and more importantly something that will affect safety. It is part of the FORD QVM program.
All the power tap points are there, shielding requirements, rear AC, body areas that you can cut etc. Great reference for all the chassis wiring as well.