We went from GM 2500 6.0 gas w/3:73 to a new GM 2500HD ExCab SB 6.0 gas w/4:10 and the six speed trans. We had very little trouble with the 3:73 on hills, and no trouble at all on hills with the new 6.0 w/4:10 ... we carry a Lance 845 at 3000 lbs dry, probably over 3500+ lbs wet.
Diesel has its moments, but for the money, unless you are planning on towing with the camper, and you said not, the extra cost for buying and maintaining a diesel don't make a lot of sense to me. You can buy a lot of gas for $8000. My truck gets 14/16 empty and around 10-12 on highway, depending on the grades. SO that's about 45/50K miles worth of gas free, sorta.
The 6.0 is a real workhorse, especially with the 4:10 ...