I have 370 hp in my 2004 American Tradition 40 and tow a 5000 lb vehicle. When it comes to the grades, I wish I had 400 or 425 hp.
The rule of thumb is a minimum of 1 hp per 100 lbs and that's exactly what I have (coach and toad combined). It's adequate but no more than that. Estimate the weight of the new cach and trailer and pick an engine that has enough oomph. If you like performance, go for 80-90 lbs per hp rather than 100.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL