What does your 84 Country Squire C weigh? Compare that to the weigh of a Sprinter-based rig. Might provide a basis for comparison.
My 2006 Winnebago View, which is on a Sprinter chassis, weighs about 10,000 pounds and I don't consider it under-powered. Even with our 2,800 pound Mini Cooper toad, it does very well. We typically enjoy 14 to 15 mpg with the toad, driving between 60 and 65 mph.
And welcome to iRV2 . . . great place!
Warren and Debbie
2014 Tiffin Allegro Breeze 32BR