I have a 1996 M/H on a P-30 chassis and had the similar problems. For the vibration in the seats I would also look at the roundness of the tires. Out of round tires will cause that kind of vibration and wheel balancing won't help. You can check by jacking the MH up with the wheels off the ground and put a block of wood under the tire and spin the tire and see what the gap is from the tire to the block of wood. Mine were 1/8 out of round and new tires fixed that problem. A double layer of insulation fixed the engine noise from the dog house. Headers greatly reduced the engine heat issues. I found that advancing the timing from 4 degrees BTC to 9 degrees also helped and added to the performance as well.
