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.
My wife hasn't used the inverter to power up her hair dryer, but we have used the microwave. Both use about the same amount of amps. The microwave is only on for a minute or less, generally 15 seconds or so. The micro was wired that way when the coach was built and I never changed it. But, we use the micro very sparingly on inverter power.
Hair dryers can take up to 1,500 watts, or about 12 amps at 120 volts. That is a lot of power. Now, if you have four (or more) house batteries and an inverter that can provide at least 15 amps at 120 volts, you'll have enough for a few minutes of hair drying time. Your wife is probably wanting to use it for several minutes or more, which could really draw down your batteries. But, then you'll have to recharge the batteries anyway, so why not just run the dryer off of the generator?
Good luck. (And, and you indicated, happy wife equals happy life.)
I also have State Farm and have been with them since 1968. They are the largest causality carrier in the country, with 6& of the business. They normally have the best rates but a local or regional carrier could give you a better rate.
USAA has the best rates, but you have to have a military connection to qualify. Also, they act as a general agent for another company on RVs, and don't write policies themselves.
Ignore the advertisements on TV over the best rates. Look up the ratings in Consumer's Reports. You can find a copy at your local library.
Since you are buying from an owner, I'm assuming he is not going to accept a trade. ASs for a value, the low retail figure is probably closer than anything. NADA estimates the figures based on a formula of so much off of MSRP, and not actual sales.
Typical? No. A rough rule of thumb use to be a percentage off of low retail, but the bottom line is whatever the dealer has set aside for overvaluing your travel trailer. Of course, the added amount is just added to the purchase price of your new RV. It is the easiest and simplest way of moving up, but you're going to pay for the privilege.
You are so much further ahead selling your trailer yourself, such as on Craig's List. Clean it up like it had never been used and price it right. Search the internet to see what others are trying to sell their travel trailers for and price your unit accordingly. You'll sell it, but it has to be priced right.