I'm not sure about one that old (mine is a 1991 Sunline Solaris T-2490 - see link in my sig below <G>), but the freon system in most semi-modern rooftop ACs is a sealed system. On a home AC the system has "Schrader" (sic???) valves (sort of like an automobile tire air valve) that an AC tech uses to attach his guages, freon jug, etc... but most RV rooftop units don't have the Schrader valves.
Now, that isn't to say that it can't be recharged. With the proper equipment, an AC tech (or someone with knowledge in this area) can drain the system, sweat in (weld) two Schrader valves (one for the suction line (low pressure side), and one for the discharge line (high pressure side)), and then vacuum and charge the system.
I myself have done the above last year, as I used to work in the AC trade, and had access to a vacuum pump, guages, and freon. I also had Dometic send me a service manual (which listed the proper pressures/freon amount). Hehe, my rooftop unit had developed a leak on the suction line, due to a bundle of wires that feed the blower motor rubbing against it for many years <G>. I had previously joked with my brother-in-law about it (who is still in the AC trade, and who loaned me the equipment to do the job)... "Here! You are in jail, and here is a piece of fiberglass wire insulation... Now, saw your way out!" <LOL>!
Anyhow that, in combination with cleaning the evaporator and condensor coils breathed a bit of new life into my old rooftop unit -
Plus, the Schrader valves that I installed allow me to check system freon pressures, or add more if the need arises.
Anyhoo... not a simple process, unless you're experienced in copper welding, and have access to the equipment (guages, torches, freon, etc...).
P.S. You can clean the evaporator coil using "Simple Green", and the condensor coil using one of the commercially available foaming condensor coil cleaners (I bought a can at Home Depot, but don't remember the brand name of the cleaner offhand). Use a foaming cleaner that doesn't need to be rinsed off though.
P.P.S. Note that a rooftop (or home) AC unit doesn't use the same type of freon as an automobile's AC system, and unless you're brother-in-law works in the AC trade <G>, you might have a problem getting freon.
One problem that I ran into with my rooftop unit, was that absolutely NO ONE (RV repair places, home AC service places, etc...) wants to work on them, but rather opt to "Well sell ya a new one!"...
A) Wasn't an option ($$$),
B) A total waste of money, when all I needed was to vacuum and recharge the system.