Yeah, that $25k budget limit means you won't have the good ones to choose from. My choice would be a 2011 Ford F-350DRW CrewCab with diesel engine and at least XLT trim (to get cruise control, AC, power everything, and decent upholstery), but according to NADA Guides, those retail for about $35k in clean retail condition.
2011 Ford Super Duty F-350 DRW Crew Cab XLT 2WD Prices, Values & Super Duty F-350 DRW Crew Cab XLT 2WD Price Specs | NADAguides
Back off one year to 2010 and choose a V-10 gasoline engine instead of diesel and you're there. NADA clean retail for a 2010 F-350 DRW XLT 4x2 with the V-10 engine and 60,000 miles is $25,500. Just a tiny bit of negotiation will probably get it down to $25K or less.
The V-10 is a good bullet-proof engine that will work just fine for a full-timer's tow vehicle. Not as good as a diesel, IMHO, but lots of V-10 owners shout their benefits to the rooftops. I would not buy a SuperDuty before 2011 model year with either a diesel or the 5.4L V8 gasser, but the V-10 is fine if you can put up with the gas mileage of a big-block gasoline engine. But the V-10 is rare compared to the diesels, so you may have to look hard to find one equipped the way you want it. Upgrade the trim to Lariat or King Ranch and you'll pay more. Insist on a 4x4 and you'll pay more. But you don't really need leather upholstery and 4x4 to tow a big RV trailer.