It's almost nonsensical to use the word "invest" and "RV" in the same sentence, but I understand what you are asking.
120k miles is not a big deal for the base engine, but everything else on that coach has 120k miles and 20 years of wear and tear. An 20 year old alternator or fuel pump or radiator with 120k miles on it is just as good/bad on a diesel as on a gas engine.
So, condition is everything. Check it out in-depth and if all seems to be in operating condition and appearances suggest it was lovingly maintained, then $20k is a reasonable price for a used diesel motorhome of that age. If you are new to RVs, get someone with more expereince to help you check out the house and chassis systems. Better yet, hire a mobile RV tech to go over the appliances and systems, and pay a diesel chassis shop to take a look at the engine, transmission, suspension, etc.
And check the sidewall dates on the tires! Tires in the vicinity of 7 years old are very likely to fail soon, regardless of tread or appearance. They are in their old age and no longer reliable. Tires that size are a very expensive item to replace. So are batteries for house and chassis.
Former owner of 2004 American Tradition and several other RVs
Home is in the Ocala Nat'l Forest near Ocala, FL
Summers in Black Mountain, NC