@Gary RV_Wizard already mentioned it but 1hp per 100 lb is about right.
The max gross for my MH is 26k lbs and we tow a 4k-ish car. Our actual weight on a scale is 23,000 lb, so we have just under 30k lb going down the road with a 300 hp engine.
We've never had an issue merging onto a 70 mph...