i'm looking at a used trailer and it has an onboard Onan 3600 propane generator. I'm not familiar with the propane ones at all. She has 2 10 gallon propane bottles on it. How long should it run with 20 gallons of propane? It is a 2005 exiss 12' sw and i live in south louisiana so if the generator is on it will be running a/c the whole time. I don't need it to run several days or anything i just need it for long day barrel races or maybe for over night when just staying 1 night. If staying longer i plug in anyway. 

I have the exact same generator and mine lasts 5-7 hours on one propane tank depending on load .

That's the 4K inverter but has 3600 because it's 10% less power with propane. 2 10 gal is not much. I would be surprised if you got 24 hours of continuos run at a load. Get a couple of 20 lb tanks to supplement. But I see now you said 10 gal, not Lb.  If it's truly 10 gal tanks then they will last a while. How much run did the previous owner say?

I have a propane generator with a 15 1/2 shortwall sitting in the full sun it will run 32 hrs with the 100 Lb. tank

That should be more than enough fuel for just an overnight trip.  I get prob. 30hrs out of our two 10 gallon tanks.