For campers weighing over 4,000 lbs. you will need either DRW or 19.5 rims and tires to support the weight. Most of the campers sold do not require a DRW truck but all trucks do require some modifications to support a camper load safely.
Diesel is gross overkill for a 4,000 lb. or even a 6,000 lb. load. For some reason people who would be OK with a gas V-8 for towing a 8,000 lb. boat and trailer think that they need diesel for a load half as great when it is a camper.
A gas V-8 long bed SRW truck will work for campers up to 10' length and all you forgo is a dry bath. Check the Wolf Creek and Lance and other camper manufacturers websites and look at the published weights for their campers and add 1,000 lbs. to have fully loaded weigh for the truck to support.
The 2500 and 3500 trucks usually have the same rear axle and wheel bearings and the only substantive difference is in the number of leaf springs in the packs and this can be altered with the addition of SuperSprings in about one hour of your time. On the used market there are ten times as many 2500 trucks to choose from and I found that the majority of the 3500 trucks for sale had either the regular cab and 2WD or a crew cab. A long bed truck with a crew cab has the worst possible turning radius and is a handicap with a camper when traveling on backcountry roads or even getting into tight campsites in USFS and BLM campgrounds.
The absolute best deals I have seen are from people selling a camper on a truck as a package deal. Not unusual to see a 10 year old truck with less than 80k miles with a full size camper selling together for $16k to $20k. Best part is that you get the tie-downs, heavy duty shocks, camper wiring, and all the other add-ons that you would have to do and pay for with a new truck and a camper.
Campers also have greatly increased in price over the past 10 years. A camper that sold in 2004 for $12,000 probably sold in 2008 for $17,000 and in 2014 is selling for $24,000. A 3-6 year old camper will save you a lot of money.