Do NOT let them repair it.
You can get an engine in a box with a FULL factory warranty for FAR less than $21,000. :E
I haven't priced them in a long time since I bought one but I paid 3,000 for a Target engine in a box complete with full factory warranty. Plus a reasonable price to install it, because it's complete they just drop it in and hook it up.
I'm sure the cost has risen since I last bought one but certainly NOT any where's near $21,000.
Call around to other repair shops and ask them for an estimate for a Target or whatever company now offers complete crate engines. They will be less than what the oem manufactures will have to charge you.
Then tow your RV out of the shop it is in and to the repair shop that will do you right. A repair shop that doesn't tell you about being able to buy a complete engine is not where you want to do business.
Good Luck. I would drop a new engine in it. :C