Just returned from 4200 mile trip with my '09 450/Suites (27k gross) and averaged about 8.8 overall. Also, just turned 186k on the truck. Ruined the trans at 150k due to running it after the torque converter lockup quit working--my fault for forcing it. Fan clutch (warranty), fuel pump (out of warranty), some minor stuff. Deleted the DPF at 98k.
I think the '09-'10 450s would be the best choice in a used unit--and if you want the Navigation, the '09-up is way different than the '08-back.
'05 36TK3 Mobile Suites
'10 F450 new to me
'09 F450 Lariat 4x4--died