1 ton SRW here. The camper I haul is around 4000 lbs fully loaded. My F 350 long bed 4X4 diesel handles it well with a set of tork lift stable loads and a stock sway bar. I would never even consider a 6 foot bed. To me a truck bed is 8 feet or it isn't a real truck.
What a lot of folks fail to mention is all of the associated costs that come with the TC. 20 grand is not going to get you far.
A 15 year old truck is more than likely going to need significant upgrades to the suspension, and don't forget about the bed mat ($100), the tie downs and stable loads ($1,000) rv insurance ($250) truck and possibly even camper registration($500).
So what your left with is more like 17 grand or so. If you can find a quality truck with updated shocks and struts and healthy tires for 10 grand I would be very surprised to say the least. That would leave you 7 grand for the camper itself. More than likely a camper in that price range will leave an awful lot to be desired. Batteries , solar system , generator etc .