You are losing a lot of payload with the diesel. I have a F350 Lariat long bed crew cab with the 7.3 gas Godzilla, 5th wheel prep, 10 spd transmission. My payload is 3921 lbs. I pull s 2022 Reflection 150 series 295RL, GVWR of 10,995. We considered the 303, but did not like the bathroom layout. I live is Mass, and vacation in Maine, NH, Vt, and finger lakes region of NY. I am very happy with this combination. I have 3.73 rear end, the tow rating is 16,200 (but you run out of payload first!). I would not hesitate to pull a 303 with this truck. Anything bigger, I would seriously look toward a diesel. People have said I need a diesel to go through the Rockies.....maybe so, but I live in Mass.....I travel at 60 mph, tow/haul mode on, and use cruise control. I get 9.5 mpg average. I have never had a situation where the truck would struggle to maintain that speed. In the White Mountains of NH the tranny shifts as needed, on larger hills it usually downshifts to 6th, occasionally 5th, but I let the truck do it's thing. (7th gear is 1:1, 8th, 9th, & 10th are overdrives). I will also say that it does a decent job downshifting as needed for engine breaking to hold me back on downhills. Would a diesel do better? Absolutely, but is it worth losing 900 lbs of payload, and spending $10k more for that option? Not for me in this part of the country. Just my opinion.