I know the temp drop figures for my home a/c & my TV's a/c, but not that on RV's. I'd bet it's available in a Google search though.
The bad news.... Hope it doesn't need recharging, this isn't like adding a freon shot to your TV's underhood system
Most RV a/c units were not designed to be recharged and don't have the necessary fittings to do so. This means that it has to come off the roof, new fittings soldered in, the unit recharged and then reinstalled. When a member of another site I am on had this done the bill was close to $400. And remember, everything else in the a/c unit, fan motor, electronics, relays etc, is still 30 years old.
The good news: For not much more you can buy a new unit and ferget about it for another 30 years (maybe...)
And, as an alternative: After 30 some years the evaporator and condenser may just be covered with oxidation and general crud and may respond to a good cleaning. There are special sprays available through a/c supply sources that do a fair job of this.
And Kansas in August without A/C??? Ouch.....