I have a Grandby with a DIY interior.
1. I have a MaxxFan in the back; I prefer it (over Fantastic Fan) because I can leave it open in the rain.
2. I found the rear window useless. I covered it and built a clothes cabinet in that space.
3. I use a backpacking stove to boil water in the camper, all my cooking is outside. I prefer more counter space.
4. I built and wired my own solar charging / battery system. Get a good battery monitor.
5. -- no experience --