I would contact a good A/C Heating specialist, explain what has happened. There is a data plate on the unit, so it shows how much Freon/oil is needed to recharge the system.
A lot of RV Techs don't spend the money on a "gauge set" because they don’t want the liability of working on a sealed system, and some states make you have a license. Calling a Heat/AC guy, he has the license, and the necessary equipment to fix this. However, if that is true, then that Freon went someplace, it may have a leak (sounds fishy to me) but not impossible since age is involved.
To fix the unit, first he will have to pull a vacuum for about 4 hours to evacuate all the air/moisture that is now in the system, and then he will refill and patch the leak. If it was me, I would call the OEM customer line, and get from them the exact replacement model. They are going to want the Model Number/if any suffix/serial number, basically just take a picture of the data plate so you have all that info at hand when you call them.
NOTE, some manufacturers have stopped making RV A/C's because the Freon requirements/specifics have changed and the tooling to upgrade the plants was more expense than profit received from making a/c's. Dometic is still doing it, but Carrier has stopped, and I think several others have curtailed their units. I don't think you will have much trouble matching the old one up with a new one.
You can change the unit out yourself if you have any mechanical skills, but will need some help getting the old one off the roof and the new one up there. You can download the installation directions from the manufacturer first so you know what's involved.
You can send me PM and I will call you and we can talk about it if you wish. I'm in Washington State.
Monty & Janet - 2007 Alpine APEX 40 MDTS
S/N - 75715 - Retired - Master Certified RV Tech