Welcome to the Truck Conversion forums! Sounds like you're looking for the HDT (heavy duty truck) crowd though. There's quite a good size group of people that use HDT's to pull their 5th wheels. They have a very active forum over here:
HDT - Escapees Discussion Forum . There are also usually a couple of very nice trucks for sale there too. If I were looking for something like that for sure look for one that's all built up and ready to go. You'll save a ton rather than converting a used tractor for rv pulling duty. Check the
HDT Rally page too. There you can find lots and lots of pictures of rigs so you can see some of the possibilities. Don't rule out a Truck Conversion either. Sure is nice cruising down the road in a single unit where everybody can wander around, sleep, watch tv, eat, play games at the table, use the restroom (all use is mandatory sitting while under way), etc....