Its very likely they are using a taller gear ratio in the rear to help with the heavier coach. Your ace may of had a 4.88 or 5.13 gear set the new one could very well be 6.xx
Not saying you don't know what your doing but do you have it in D or is tow/haul mode engaged? If it's not in drive it won't go to O/D and tow haul will delay it going into O/D.
If you are in fact getting into high gear I would not worry about it. I would just adjust my cruising speed accordingly.
2016 Jayflight 32 BHDS Fiberglass sides. 16" wheels and LT TIRES
2017 Ford F-250 CCLB 6.2 with 4.30 gears
Traded06' Chevy CC Dually 6.0.