Keep in mind that when you do take a test drive, it basically will just tell you if the driving position is comfortable. All RV's ride rougher when they are empty and rattles are not too surprising.
Check to make sure that all of the controls are within your reach and that the seat will adjust for all possible drivers. Listen for any unusual sounds from the engine and drive train. Make sure that the transmission shifts as it should.
If you are serious about a unit, ask to see the service records and check to make sure that any recalls have been taken care of.