First, I would not buy anything Dutchman or THor, but that is my preference.
Next, for the truck selection, I would use the trailer GVWR for the proper selection. You will be surprised at how fast the "stiff" adds up over the trailers dry weight. The dry weight in the brochure will not include any options. Anything that is listed as an option will add weight. This is the A/C, microwave, TV, batteries, full propane tanks, awning, etc. So before you add anything to the RV you can be 1000# over the dry weight.
FOr a 15000# GVWR trailer, if you do not load to heavily, maybe a 1 ton (350 or 3500) SRW (single rear wheel), but you will most likely need a dually. In any case, a diesel is the preferred engine.
And 5er do not pull like a sled and beat you to death. If you get the right truck, and a trailer with a good suspension system, they do fine. A spring suspended motorhome can ride like a buckboard with square wheels.
Ken