Cliffy49,
my younger brother just bought a leftover 2020 gmc 2500 with all the bells and whistles. it even has trailer tire pressure monitoring. it has the 6.6 gas and his tt is around 5k pounds. he and his wife took it from northern delaware to inglis, fl. for their anniversary. he said he got around 11 mpg. which is pretty good since the truck isn't broken in. if i were to buy a truck now, i would go for gas. good luck with whatever you decide, oh, by the way, he was a ford guy, which is not a bad thing. if you ask me, i don't think you can go wrong with any of the big 3, jmho.