While the question appears to be one of the legal or licensing problems with length, the real question here should be of overloading the chassis on the coach. Brakes, radiator and suspension limits need to be addressed.
The three most popular Cummins engines are the ISB, the ISC/ISL, and the ISM. You'll need at least the ISC/ISL and the ISM would be better. Unfortunately, the ISM is available on only a few top end coaches. The ISC or ISL with the side radiator on a chassis with full air suspension and air brakes would be a good choice.
As for using a gas powered coach, I wouldn't recommend it.
As for length limitations, I've never heard of anyone in a motorhome getting stopped or checked. You won't need another license if the license you have in your state is sufficient.