My two cents:
If you intend on never upgrading to a larger camper or larger tractor you will be fine with a 1/2 ton, but if you plan on upgrading then a 3/4 ton makes more sense. If you go with a 1/2 ton find one with more than a 4 speed auto. An extra gear or two keeps the engine at it's peak power output. I had a 2001 Silverado 4x4 with a 5.3 and 3.73 gears. I liked the truck and it got great mpg when not towing at the design limits. I pulled my TC40 a few times with it. The truck worked well when the terrain was flat but it struggled pulling up the mountains. It was also used to pull a 6,000 lb tow behind camper. The truck struggled to maintain 55MPH on I-81 and in my opinion there was just too much of a gab between 2-3-4 for the engine to operate without pulling 4k on the tach for long periods of time.
The diesel option is worth it if you find a decent deal, but you have to look. Right now is not the time to find decent deals. I bought an 07 Dodge 5.9 back in 2008 when the big 3 were getting ready to close their doors and diesel was over $4 per gallon. I bought my truck with 13k on the odometer and 10k under book from a dealer. My father purchased my Silverado and he upgraded to a larger camper and he found a 2007 Silverado Duramax with the 6spd Allison LT3 for under 30k with 22k miles. I have looked for 2006-2007 Dodge Mega Cab (my quad cab is tight with three kids) and a Crew Cab Duramax during the past year for my growing family, but these trucks are few and far between with premium prices if found (pre-DPF & Urea). The 7.3 Fords with low mileage are also hard to find.
I believe I would look at a newer 3/4 ton with a 6spd auto. There are a lot of diesels parked at my workplace. Here is a list of the following problems encountered:
2003 F350 SRW 4x4 7.3 Lost an injector at 90k (I believe ULSD related) I finally talked the owner into using an additive.
2004 F350 DRW 4x4 6.0 50k it lost a head gasket while pulling a horse trailer
2001 2500 Dodge-lost a fuel pump at 160K
2001 Silverado 2500 LB7-lost an injector covered under warranty
2008 Dodge 3500 6.7-DPF problems, 14-15mpg
2008 F250 6.4-horrible fuel mileage DPF related 14.5 mpg average
2004.5- Dodge 2500 5.9-no problems as of 50k
2007 Dodge 2500 5.9-no problems as of 29k
2005 2500 Duramax no problems 70k-owner did slip tranny with an Edge programmer
2006 F250 6.0-40k no problems
2006 F250 6.0 60K no problems
2006 Duramax with Allison-no problems other than people running into his truck in parking lots.
Gas Trucks
2006 1500 5.3 Silverado-owner pulls a 8lb tow behind camper and admits the truck is a it's limit. I believe he also has a 3.55 rear end. The camper pushed him off the road during a rainstorm when he had to stop suddenly on the interstate. No one was hurt.
2004 2500 6.0 4spd- owner pulls a 8lb camper and gets 7mpg while towing. Not a problem until you pull across state lines and have to stop often. He complains about the lack of extra gears also from 3rd to 4th.
2005 F150 lost it's transmission at 147k miles and routinely pulls a 3k camper.
2007 Titan-rear end problem- The rear limited slip has caused problems and the warning light comes on and stays on if it engages. Don't know what the outcome will be since he hasn't taken it to the dealer yet.
2008 Dodge 1500- 28k no problems but it's a daily driver and never tows