I can highly recommend the BMW X5 35d, model years 2012 to 2018. If possible, get one with the third row seats, as they also have rear air bag suspension (self-leveling). The diesel is no longer available for later model years. For those model years, the maximum tow weight is 6,000 lbs. and the turbo diesel drags it through the mountains easily. The 3 liter twin-turbo 6 puts out 265 HP and 425 foot-pounds of torque.
If you do decide on the BMW, be sure to have the official BMW trailer wiring harness installed, as the aftermarket harness trailer companies push screws up the vehicle electronics.
I have taken my 2012 with a loaded trailer at about 5,000 lbs across the rockies and back west of Denver twice as well as many times over a shorter mountain (rise of about 2,000 feet) here in the east that has a 15% grade on both sides. In 90° F weather.
As to the cost of fuel, the diesel gets about 1/3 farther on a gallon of go-juice than the corresponding gas version, so as long as diesel fuel in no more than 1/3 more expensive than gas, the diesel is still cheaper to run. Today, gas buddy reports diesel at $5.49 and RUG at $4.49 at Sam's Club, which is 22% difference, thus diesel is still cheaper per mile.
One thing to note is that coasting in gear, the ECU shuts off all fuel to the engine. This if you coast down a long slope in cool or cold weather, the engine will be stone cold by the time you reach the bottom, so don't be surprised to see a puff of black smoke or a check engine light (not to worry, goes away after a restart) when you finally step on the go-pedal.
Driving the Jeep diesel I had before the BMW, an idiot tail-gated me all the way down Going To The Sun road at Glacier NP in 50 degree weather. I have no idea what he thought he was doing, you can only go so fast due to traffic. He stayed so close I couldn't see the top of his grill. I was delighted to watch him vanish in a cloud of black smoke when we got to the bottom. I think he entered another dimension, as I never saw him emerge from that cloud.
