My advice to you is to go drive a few of each option. See what fits your expectations best, from the interior arrangement/materials, to the responsiveness of the different engine options. What fits in your garage. And so on.
As mentioned, the Cayenne will be more expensive to buy/maintain (and possibly insure), but is said to drive a bit better (not likely a primary concern of yours given annual mileage expectations) when pushed. A Q7 is a long wheelbase version which may suit you better given the towing element. Not sure how many folks or how much cargo you want to haul, but that could potentially be an important point to consider. A Touareg will be roughly 85% the same as a Cayenne, with the same shorter wheelbase; essentially just a Cayenne in sweat pants instead of a tuxedo. Although I much prefer the 7P interior to the Q7 (until 2020, then it's miles better), or the Cayennes. Which is strictly personal preference. The VW will cost a margin less to buy and have/keep maintained, even if many of the mechanical parts are interchangeable.
People here will tell you to go with their favourites, it's human nature to enjoy having your biases confirmed.
I'd wager in the conditions described you'd be fine with a TDI, and that's what I'd recommend. Tough to have too much torque, or too good of mileage. Main thing would be using VCDS or similar to monitor the chain stretch values and the DPF stats, having fresh DEF available, maintaining fresh fuel conditions (biocide for a TDI, fuel stabilizer for a gas engine/no ethanol in the tank during storage), annual oil change regardless of mileage, keeping the battery topped up, and preventing the tires from dry rot/flat spots, etc.
Not sure what Oregon is like for smog, if you're legal/willing to delete that's always a good option and your EGR issues become moot. Failing that, an eco tune like Malone's stage 0.5 will go a long way without having to do the full delete. And for any of them, a transmission tune by yrktreg will improve your driving characteristics while driving and especially towing (IE regaining engine braking).