we have a 5th wheel that weighs ten thousand pounds loaded . we pull it with a 2013 Ram 2500 HD with the 5.7 hemi and 3.73 gearing
does a good job . flat terrain I can cruise 62 MPH turning 2000 RPM
hills it downshifts as it should , but the 6 speed trans takes care of the downshifting with ease . we get about 8 to 9 MPG towing . we just did a 1000 mile trip to the outer banks and back .no problems
yes , diesels are great for towing, but the extra cost for a diesel can buy a lot of gas . I think you will have no problem pulling that size 5 th wheel. good luck what ever you decide.