You will find the coach to be severely under powered. I had a 40' Dynasty with the 300HP Cummins. It couldn't get out of it's own way if it tried. I hate 'abusing' my equipment and it always felt like I was doing that with every littl hill or mountain. It struggled, that's for sure. That said, how much driving do you plan on doing? Most spend more time camping than we do driving so that choice has to be yours.
In summary, YOU are RIGHT! The coach will/is underpowered. Now if you want to ADMIT you're right to the missus....well that part, my friend, is up to you..:E....Dennis
