I live full time in my bus and I didn't insulate much - The walls and ceiling were factory insulated - and still in decent shape.
The floor is not - I didn't do much to it to help, either. It's fine, except in the winter, then it's really cold - but a few carpet pieces help a lot.
I left all the windows - and those are my main source of cold air in the winter and heat in the summer - a roll of reflectix helps. RV windows are certainly better - but I am one of those who likes the bus window look.
A Mr. Buddy heater will heat up the bus nicely and take the chill off. A Cubic Mini Woodstove really heats up my bus.
I have screens on 6 of the windows - and those stay open most of the summer - with a few small fans going - it's nice. But, that was not the case in Florida- where it was hotter than a pepper sprout. I'm was in Montana last summer - and I am back there now.