If money were no object the solution would be fairly easy- a new Ram or Ford dually in their maximum towing capacity configuration would work. Since your budget is $15K or less you have some challenges. Here's some options as I see it:
Older F450/550- If you can find a flatbed diesel that's in decent shape for under $15K this truck would work. I'd avoid the 6.0 diesel, and anything newer will likely be over $15K. As you know the 7.3 is a good motor but even with 4.88 gears or whatever an F550 would come with towing 28,000 will make for a slow ride up hills. If your 20' work trailer is over 10,000 GVWR you'll need a CDL to be legal towing with an F550.
School bus converted to tower/hauler-
Actually not a bad idea if you are handy and can DIY. You'd want one of the "short buses" built on a HD chassis, an E350 chassis bus would not quite be up to handling the pin weight for a 28K fiver. Most of these only have about 200 hp and aren't exactly fast as a bus (I've driven quite a few). They would be dog-slow in the hills pulling 28K but a HD one with air brakes would do it safely, just very slowly. A HD converted bus if over 26K GVWR would require a CDL to use for work. If just towing your RV you could probably get away without it.
Class 7-8 hauler-
No concerns with weight, towing 28K is a joke for these guys. If using for work then you're dealing with all the commercial vehicle headaches. In an F550 you might sneak by the scales but not a chance in one of these big boys. You could probably adapt a short flatbed to one fairly easily for your camper and either convert your fifth wheel to a gooseneck or install a removable hitch. A big truck in your price range will likely have a ton of miles and be a full manual transmission, which there is a bit of a learning curve if you've never driven one.
