@flintknapper, do you know how many BTU's the stock condenser and evaporator are rated at?
No. Have never found any published data. But I can tell you that for a vehicle the size of an 80 series a minimum of 18K-22K BTU would be required. Recognize...that higher rpm (highway speeds) this same system (variable..since engine driven) might produce 40K BTU or more.
For a rear air unit (mean't to work in conjunction with the primary and sharing the same condenser).....around 16K would likely get the job done.
You'll find that when adding a rear unit...simply 'moving air' around makes a big difference for Humans and Pets. If you can provide them with circulating air 25-30°F lower than ambient (you might get more) you are going to be somebody's new hero.