At this point you’re going to have to go and get the manual for diagnostic steps. There may be one problem or more than one. The procedure is repair the first in line and go from there. Do not assume the gas cap is the problem; there is more to the evap system than that.
The dealer is not being “slippery”. No one in their right mind is going to flat out tell you what the problem is without having the car in hand. More than once someone has been told something off the cuff by the dealer and the dealer then becomes the recipient of a cussing when that something does not work or someone screws something up.
Another reason is that the service dept. is in business to make a profit. It would be very easy for the dealer to spend 10 hours a day dishing out free advice and chances are overtime would kick in when the word got around town about a free diagnosis being done. Shortly, there would be no diagnosis of any sort because all of the mechanics would quit since they’re paid on flat rate and free diagnosis means no shoes for the kids.
None of this means the car is a headache. It’s a 5 year old used vehicle and things happen. Can it be ignored with no bad side affects? Probably. It’s up to you and any emissions testing station if that is a factor.