Hello PV RVer and welcome to iRV2. You will probably get more attention to your request for input if you post a message in the Thor/Damon owner board or Motorhome General board. Not everybody reads the Introduction section here.
The "rule of thumb" is that you want at least 1 hp per 100 lbs of weight, so a 275 horse engine should move a 27,500 lb coach (don't forget the toad if you will have one) adequately.
You can use the SEARCH function to find the several previous discussions of the 5.9L "B" block problem.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL