"We are around 25mph when we reach the top."
IMHO:
If the best you can do on a 6 percent grade on the interstate is 25 mph you have waaaay too much Toad and/or not nearly enough power for the load you are expecting it to haul.
Even heavily loaded 18 wheelers do better than that.
Class C's over 30' tend to have very little payload available and adding a Toad that is very close to your max towing capability is a recipe for disaster. You may be FunTwoDrv but probably are not much fun to follow.
As always... Opinions and YMMV.
:E