Wow! This is quite a response and a lot of experience to draw from. This forum is awesome. Thank you all for responding!
I think I've got the answers to most of my questions. The main one being that it is NOT worth it to insulate the walls well if I leave the stock windows in, because they transfer too much heat [outside when it's cold, inside when it's hot].
I am pretty confident that cold won't be an issue because heater's are easy to put in the bus, and in the case of fire sources (like pot belly stoves) very cozy and have a nice aesthetic. It would actually be nice to sit around a little fire and read on chilly nights. If it's really cold, I'll just get a goat to snuggle with
I've always preferred cold (good thing I live in Canada?) over hot days because I find it so much easier to warm up than to cool down.
Definitely more worried about heat in places. A/C's are common solutions, but I have stated my concerns about that. I actually have a friend that didn't want to be in my apartment with the A/C on due to health concerns - and I think a lot of us are environmentally conscious enough to shy away from using it often. Though, does anyone here even think it's possible to have a comfortable bus on hot days in the best case? Best case being super well insulated, windows swapped for RV windows [I was reading that residential windows aren't suitable for this application as we change elevation a lot and move a lot, but can work]. And probably some reflective blinds. Maybe this is a pipe dream.
Does anyone know of cooling methods other than A/C?
I also imagine solar panels on the roof would also help keep the bus cooler, similar to the paint? Or do they absorb heat and make it worse?