It's all about Budget Budget Budget.
If you're spending a LOT of time towing, and going to be in the mountains for a lot of it, a turbo is sure nice, whether it's a diesel or an ecoboost...
If you're towing on vacations only, and less frequently in the mountains, then a gasser will be fine, and will get the job done for less money.
I'd be leery of buying a diesel truck at this point because you never know what the jackrod before you did to it... It's far too easy to put a tuner on them and cook transmissions and head gaskets and then broom the truck when the problem shows... If you can find an old man owned truck, then it's a safer buy, but cheap, reliable diesel trucks are getting hard to find IMO.
Honestly, if your Tahoe will tow the new trailer, I might suggest that you keep an eye out for a Suburban with an 8.1L. I think it was offered up through 2006, or early 2007 for trucks... Find a vehicle from out west. If a truck would better serve you, find one with the 8.1L and the Allison trans for best trailer towing duty.
My experience with Excursions towing is that they are a handfull... Sloppy handling and soft rear suspension do not make for good tow rigs...
I would also recommend you try an Ecoboost Ford F150... You may be surprised how nice having a turbo makes the towing experience. The EcoDiesel Ram 1500's would also be a truck to drive, if you can find one...