For rain I have window ventvisors so you can open them at least 3 inches without any rain getting in. I always keep the front side windows open at least an in in winter and whatever is comfortable in summer (I also have homemade bug screens) Summer it might be 98 degrees today but it drops into the 70's or even lower at nite. If it gets hotter than that I normally stay home or head up into the mountains.
Winter camping, sleeping inside is still warmer than sleeping in a tent... But, I have 10mm thick insulation in all the doors and roof and my windows also have an inch of insulation except for the top 3 inches of the front windows. (see my build) I would not leave the motor running over night but, I have been guilty when temps dropped into single digits to turn on a small little buddy heater for a little while to warm things up.
I have been thinking of getting a diesel heater that runs outside and I will plumb it to blow fresh warm air in one of the windows and leave one window cracked for the stale air to go out.
Feel free to ask questions and check out my build threads.
I have one on this site the link is in my signature below.
A more complete build thread is on the
Overland Bound site