I just did this (old TT to newer but still old C). Roughly speaking, I narrowed it down to an 03+ Ford chassis which guarantees the most updated V10. No worries about spark plugs or anything like that, it's a true 300k mile motor.
From there, it was all about condition. I was not so much concerned with mileage. First and foremost, inspect every corner and joint, look under every mattress, pull up the carpet a bit in hidden corners, and look for ANY sign of water leaks. This is the single biggest killer of RV's. And in C's, it's the cabover that is most suspect, but really a leak could be anywhere.
I then gave a greater preference to rigs that were privately owned, since I could actually speak with the owner and get a feel for it's maintenance history. Most I saw on dealer lots were trade ins, the dealer literally knows nothing about them.
Carfax can sometimes give you a rough idea of it's usage. For example, you'll see the mileage each time it changed hands and may be able to tell if it was driven 50k in the first two years, then sat for the next 6 years unused or something like that.
Have fun and take your time! I looked for a few months and ended up traveling out of state to buy.