3500 for sure good argument for a dually.
Diesel for sure if you want to travel more than a few short trips a year.
15500 is heavy and if you plan to drive around in the mountains you will love the diesel. If you plan to full time you will love the diesel. Because we now almost exlusively use our TC we switched from our 2011 3500 duramax to a 3500 6.0 gas. There is a huge difference in the two pulling a heavy trailer.
In my opinion a 2500 is out of the question.
Also in my opinion ask all the questions you would like. If people don't want to comment they can just go to the next post.
JMHO