You don't say new or used. There are older 34-36 DP's out there, but were not typically built to sleep four, unless you want the kids on a sofa or dinette. Newer coaches are now more accommodating and offering bunk house models.
My sister has a 2004 34' Itasca Meridian with a 350 Cat. It's rated to tow 10K.
Here's something to think about. You're talking about using it at a track, which means boondocking since you're looking for a large water tank. Many of the older/smaller DP's used a propane generator, instead of a diesel. For me, that would be deal breaker.....they're loud and go through their small propane tanks quickly. Stay with a diesel generator.