good trucks but they dont offer any big HP motor like the 6500 trucks . the duramax in the 4500~5500 truck is still only rated at 310 hp If I recall correctly. I am not saying that not enough , I am just saying if you have a tall 5th wheel and traveling into a head wind more Hp/ TQ would be nice to have. Air ride rear supension would be nice also. The company I work for has a 99 GMC top kick with a cat 210 hp and a 6 speed , 186000 miles to date. 3 set of brakes , one set of tires at 165,000 miles (on the OEM michelins) . with 210 hp I would opt for more power , its slow on hills. It has a 12' high box and weights in around 17~18000 lbs most of the time. Automatic trans suck up more power the standard shift models so if you get a auto look for a 300 hp truck. gearing makes a big differance , our truck has 3.70 rear gear with 11-22.5 tires. Its geared to run almost 90 mph but limited to 73 by the ECM. Back in 1991 I worked for a towing company that bought a 6500 6 speed with a cat in it , they traded it in with 287,000 on the clock with no issues . Tire,brakes and one clutch thats it!
Cat motor 300 hp / 850+ ft lbs of TQ in a C-6500
Duramax motor 310 hp 560 ft lbs of tq in a C-4500~5500
2001 Holiday Rambler 31' cks 10000 lbs loaded
2006 2500hd duramax/allison crew cab