If you are even the least bit handy and don't mind getting on top of your RV, this is definitely a do-it-yourself job and will save you several hundred dollars.
If the 13,500 BTU unit was doing the job before, why go larger?
Don't know best place to buy in Utah, but bought a Carrier brand 13,500 unit in Houston from PPL last week, rooftop and ceiling unit for ducted system, for $546.70, plus sales tax.
4 bolts connect the upper and lower units, then unless your rig is really old, the electrical connections are modular. One thing to make sure of is clean very well all areas where the gasket touches.
Took about 1.5 hours to install. Just watch carefully as you un-install the old one, and re-install the new one the same way and you will be surprised at how easy and quickly you will finish the job.
Good luck.