Why would my window ac unit only drop 10* regardless of outside temp?

It's an old GE window unit, probably from the 70s or 80s, but it blows ice cold and runs like a champ. The problem is, when the inside temp gets to about 10* below the outside temp, it stops. I mean it keeps blowing, but it's like the compressor disengages as if by a thermostat. It only has low-med-hi settings. I always have it on hi. If it is 79* out it will cool the room down to 69*. If it is 102* out it will only cool the room down to 92*. Then the fan just blows. Was this some sort of primitive energy efficiency tactic? Can I get in there and pull some wires or something to override it so the compressor keeps running? I can afford a new unit, but I'd hate to replace this one as it is a perfectly matching component of my restored vintage VW camper. Any thoughts on how I can twist it's arm, so to speak, and keep that compressor engaged would be greatly appreciated.