30% off "MSRP" is normal in the RV world, any less and the dealer isn't trying very hard (certain brands like Airstream excepted). I agree on buying new unless you're willing to put a lot of time into research and shopping.
As far as length and weight for a half ton, general rule of thumb is under 30' and under 6k pounds dry weight at the high end. The first big question is how much payload capacity does your truck have, it's on the yellow sticker on the drivers door jamb. Also how many people will you be hauling with the trailer? My truck has just under 1,700 pounds of payload and with 5 of us in the truck and my little 3k pound trailer and supplies for 3 weeks we're pretty much maxed out but that's 4 adult sized people and a tween.
