If you're happy with the price and the seller is happy with the price, then it's hard for me to say it's not a good price. Very much of the actual value of a used RV depends on its condition, which is pretty much ignored by the NADA numbers. Prices also vary quite a fair bit geographically; finding a decent used RV for $10K would be pretty hard in New England, from what I've seen. Well kept, practically no water damage, everything works properly...that's worth some money; and if your needs or wants are rather unusal and specific, you may have to pay more to get them met.
Be extremely careful with your towing decisions. The diesel engine doesn't change the brakes, the frame (and frame extensions), the hitch, etc. I'm not really sure it even would do much better in general with towing than the V10, though it will be running at a lower speed and hence sound "better", or at least different.
A fair few newer Ford class C's have 7500 pound tow ratings, by the way.
