The reason why many people advise settling on the trailer first is because too many folks will fall in love with a too-heavy trailer and "have to have it" while ignoring the fact that it's too much trailer for the tow vehicle they have. But really, you are doing ok because you have a pretty good idea what you want trailer-wise and are buying your tug accordingly.
The diesel seems to work better than most gassers because it tows at a lower RPM, thus it sounds like it isn't working as hard. The noise factor is a big psychological issue. But for the trailer size and weight you are considering, a large V8 or an Ecoboost will do fine. Actually the Ford Ecoboost will tow up the grades at somewhat lower RPM because of its turbo which feeds more air into the engine (just like a turbo-diesel). But in the end, whether at 2000 RPM (diesel) or 4000 RPM (V8) you will make it to the top of the hill just fine.
One thing I would make sure the SUV had is a transmission temperature readout. Not just an idiot light that comes on when the trans is already too hot, but an actual readout. That way the driver can monitor the temp on those long climbs and slow down if it's rising too much. Excess heat is the transmission's big enemy.
I like the list of vehicles jerem0621 gave you. Some good choices there, and no need to buy a diesel if you don't feel the allure.
Today's diesels have complexity not seen on the diesels of 10 or 20 years ago. The emissions systems include particulate filters that can clog unless they regenerate regularly, and short commutes can be problematic for regen's. Many if not most new diesels require special low-ash synthetic oil that isn't always available in stores, so you have to buy it online or from the dealer. The MPG advantage is offset by the higher purchase price and often higher fuel cost, so payback takes a long time. Repairs on an aging diesel tend to be higher than for gassers; Chrysler will sell a lifetime max care extended service contract on a gas model, but not on a similar diesel unit, and there must be a reason for that. You have to get used to finding stations with diesel and remembering which nozzle you're grabbing... if you have 1 gasser and 1 diesel, you have to pay attention. And then you have Diesel Exhaust Fluid to add into a separate port, often located next to the fuel port... don't refuel with the brain on autopilot, because a mistake will cost thousands.