Run, Toto, run!
PDI should be free - it's their responsibility to ensure that the coach is clean and defect-free before delivery. Anything they find and fix is covered under warranty (assuming a new unit) and will certainly have been billed back to Thor. Charging you is double dipping.
The dealer will want to do the PDI in an hour. To do it well will take 4 hours minimum.
Fees for filling the tanks are not unreasonable. But, if the fee they are charging is out of line with reality - demand they deliver it empty. No propane, 5 gallons of gas.
From your note, I suspect that you've already paid for the unit? If so, that's too bad. You were ripped off to the tune of about $1900. (Assuming that gas and LP tanks really are full). "Processing Fees" are roughly equivalent to "Additional Dealer Profit". The dealer will probably have to pay a small fee for title processing, but nothing near the $600 they charged you.
A friend of mine always carries a clipboard when he buys a new vehicle. On the back, he's taped a sign that says, in big black letters, "A reduction of $500 additional buyer savings will be applied to the negotiated price of any vehicle purchased." He's never paid the fee...
I recommend not signing anything until after the PDI is complete. That way you can demand inclusion of any/all issues as dealer "we-owe" items in the contract. When you do that you put the warranty hassles back on the dealer. Makes life much better. My dealer didn't like doing it that way, but when they understood that I'd walk otherwise they gave in. This is probably not possible if you are not buying a unit off the lot.
Regards,
Randy