Search the specs for the kind of truck you're thinking about. You'll see "BBC" which is Bumper to Back of Cab. For a Peterbilt 379 I think its 132" or 11'.
Good luck. What kind of software are you using for the drawing?
Started looking for 379 Peterbilt TC, 24' to 30' box, bumper pull--but ended up w/1999 Liberty Coach conversion of 45' Prevost XLV bus. 1,000sf heated/AC'd race shop w/dump station, 50amp shore pwr where bus parks, 3 NASCAR/ARCA race cars & 26' Bravo trailer.