I'm using the stock 12 gauge roof wiring on my Eagle shell. It works. The part outside the roof to panel is 10 g.
My under the rear connector is there for the same reasons you want. So far it's only been used to test it. It's 8 gauge marine wire. I'm not going to rip the camper apart to replace wiring that works. But anything new I add is up sized and marine spec.
As for the shower tube. I haven't used it on the camper yet. 5 gallons is a lot of weight to lift with the roof. I need to upgrade to exterior lift struts before I use it on the camper. I was surprised how much just the racks and solar panel increased the lifting effort. I've upgraded my inside struts from 60-80lb. It helps but I'll need more for that shower.
