I can't answer all your questions.
1) I don't think a roof top carrier will work as you will need a ladder (not the one on the back door) to use it so now you get to figure out where to carry that. I don't use my kayak racks for the same reason. I've put boats up there, but it ain't fun and you need a big ladder at all times.
2) So if you remove the racks, you will have more room for solar panels.
3) I've dedicated the lower left hand side area behind the sliding white doors for my electrical cords, adaptors, water hose and chemicals. I use the supplied crappy dump hose on the road and a real one at home, and that one stays home.
4) Look into getting an accumulator for the water pump if you feel it runs too much. It's not an issue for me but if it was I'd find room for
this.
Welcome to the forum,
Eric