For over 10 years I have been reading posts on this and other RV forums. Most RV'ers with 5th wheels report between 6-12 MPG while towing and fully loaded for camping.
I have also read many posts from RV'ers who have tried to improve their towing MPG. Aerodynamic add ons, more efficient fuel/air delivery systems, more efficient exhaust systems, different gearing, different tires or air pressure in the tires... The results from these efforts result in at most a 1 to 3 MPG improvement. I've read about folks spending $2500 to $5000 in an attempt to improve MPG only to get 1 to 3 MPG more.
In my opinion... RV'ing is an expense that is worth every penny. There are hidden savings... better mindset, health, exercise and quality family time.
Don't worry about the cost....enjoy the ride.
Buy a factory made 5th wheel travel trailer and start the adventure. Don't waste time and money in trying to put something together unless it's your dream to build something out of a horse trailer.
Just my opinion and I wish you luck in this new adventure.
By the way I have a Chevy 4500 with a Duarmax Diesel. I pull a 7.5 foot wide X 12.5 foot tall X 38 foot long 5th wheel that weighs 15,000lbs. I average 10 MPG while towing.