The Ford V10 is a good motor overall. Early ones had some trouble with spark plugs being ejected if not carefully torqued down during installation due to a bit of poor design of the head (only a few threads holding the plug in), but that was resolved after a couple of years. With proper care and maintenance, the Ford drivetrain is pretty much bulletproof and likely to outlast the house part of the motorhome.
IMHO condition is more important than either age or milage, though of course all are related to a degree. Do check very carefully for leakage in the cabover section of a class C; this is a very common weakness, and the ensuing damage can be quite costly and/or time-consuming to repair.
