for the amount of miles the average guy puts on a Class A there is not much difference. It is not uncommon to find 10-12 year old coaches with less than 50,000 miles on them.
You probably will not travel much over 65 so how fast isn't a criteria. As far as going uphill. If it takes 30 seconds longer in a gasser to get up a steep grade and you are in that much of a hurry then; you should find other means than a class A to travel. Think comfort of a class A speed of a corvette. two different things both nice but not the same.
If you were full timing or snowbirding then you might want a big DP for the luxury of home but for your "long distance through mountains twice yearly and camp all in the mid Atlantic states" either will serve you well. Find a solid well maintained class A if your are looking used. A poorly maintained DP is worse than a well maintained Gasser and the opposite is just as true.
Remember length is a limitation in some Public CG so if you plan any out west trips and stay in National parks etc that could be a factor