Ok open the outside panel. On the lower right (Under the round tube) look for a brown or yellow dust/rust/powder. If you see it very good chace the cooling unit (The guts of the thing) are toast. NOTE THE WORD CHANCE,
16 years it's not unexpected.
IF that is the case you have two choices.
1: total replacement (I do not suggest)
2: An Amish Built replacement cooling unit (I forget the company I ordered from)
TO do that you remove the unit remove the "back" replace the back and reinstall
When I did it with the aid of a professional it was a two person job She (the RV-GIRL professional) really knew her stuff.. We got it done (last customer of the day) Went to dinner and when she dropped me back at "home" it was already cold.