One other note - if you scroll down on this page you'll see prior posts related to camper conversions. May help you decide when you see what others have experienced.
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Insulation and heat(neither are needed in SW MI) are both moot with adequate ventilation.For winter I'd put some insulation in the wall just to keep heat in there better or maybe even have a heat lamp for extreme conditions like if it gets super cold but your birds should be able to handle the cold.
Just make sure they don't get too hot and have some air flow
About Cornishx birds: even a step one foot high will be too much for them. We had damaged legs when we had a jump like that one year with those sad Cornishx chicks. Make a ramp or something so they can't jump up or down at all.
Love your new camper though!
Mary