You don't need to tow 50,000 miles a year to justify a diesel. Try towing 13-14,000lbs in the mtns with a gasser. Trouble is theses days that RVs are just getting bigger. 15-20 years ago you didn't have the market flooded with triple axle THs and FL 40' 5th wheels. The RV lifestyle is gaining momentum with the retired crowd as more and more people are retiring they get into RV'ing. Hence the upsurge in more residential style trailers which comes with more weight. Kinda hard to hookup to a gasser and tow 14,000lbs around the country for a few months at a time.
With the bigger heavier 5th wheels and TT's the need for a diesel becomes more common.
It's not that a diesel needs to pay for it's self it more that it's needed for the weight being towed.