The biggest concern I have with using a conventional RV and putting it on a commercial truck chassis is most of them are very lightly built so they can be towed by a standard PU, not sure how long the structure and interior will hold up on a much rougher riding truck frame. You will also find a lot of hidden costs and materials that will add up, and quite likely by the time you buy a truck and a trailer and add up all the costs, you could get one already finished off of racing junk. I have been fighting the battle of build my own or buy a finished unit for some time, I would defiantly prefer to just buy one and be out using it, but have not been able to find one that meets my needs, and cost less than 200k lol.
Doc Weaver on this site has built him self a nice looking rig using a box truck as a starting point and finishing out the interior. Spend some time looking around on RacingJunk and some of the other web sites and you will find lots of different rigs for a good price in this economy.