Do you warm up camper before heading out for winter camping?

If the mattresses are foam and have been stored for awhile, they may be slightly damp which will certainly interfere with warmth when winter camping. You might bring them (and bedding) into the house a few days before you leave- that would make sure they don't have any moisture in them when you get there.

Other than that, I doubt that preheating the camper itself would make any difference since most cool off so fast even the shortest of trips are likely to end with a cold rig at the destination.