Here are a few things to consider. Box trucks such as a Fl70 or 4700 are expensive compared to a class 8. You have to go down to the early 1990's to find a good box truck in the $15,000 range. Also watch the gearing on these trucks, many are used intercity and won't make it much over 55 mph. The hieght of the box can also be a problem, you most likely will not be able to put a AC unit on the roof and still make it under electric lines and some bridge over passes. And the boxes on these trucks will require some beefing up structurally.
Easter Conversions does a TT on a class 8 --> http://hometown.aol.com/lea9352812/
Now anything can be done, but here are some additional factors to consider. TT's have to be light due to there chassis and towing issues. So the products used in a TT (and for that matter some motorhomes) have to be light weight and usually are not very durable. Also it is difficult to marry the truck to the tt and make it look like it was suppose to be that way.
You would be much further ahead if you get a good Class 8 ($15,000), stretch it ($3500), have a box built ($28,000) you finish off the interior. Transport Design will build just the box.
Just some thoughts, if you begin the process keep us posted.
2003 28' Show Hauler Motorhome on a 1995 FL 120 completely refurbished chassis. Currently in production - scheduled completion 1/10/03 www.showhauler.com