[travelin texans]

I've towed heavy, 16.5k, 5th wheels with both gas & diesel trucks & would not go back to a gasser. The power difference is amazing then throw in the exhaust brake & you're comparing apples to oranges. I agree the fuel mileage is not much different til you get into the mountains, the diesel mileage won't change nearly as much as a gasser not to mention you won't have to have your right foot shoved through the firewall to keep it climbing.
As to maintenance, I quite honestly didn't notice much difference between the two. The diesel oil changes were every 10-15k instead of 3-5k, the oil filters cost the same, the oil cost the same per quart just took more, fuel filters if done myself were about $40 every 50k miles, all other maintenance is the same for both & everyone brings up the DEF which in my case was about $10-12 for 2.5 gallons every 3000-3500 miles which is pretty cheap per mile. The other advantage if towing a large RV with a diesel is you can always fill up at a truck stop without having deal with the idiots at the gas pumps.
If you're planning on going full-time with a RV 12k pounds or more I'd recommend a 350-3500 diesel truck, depending on weight doesn't have to be a dually, over the 250-2500 gasser & hit the road.