A/C off of 20 amp service?

Ours simply kicks the Breaker off.

20A is definitely borderline because of the smaller wiring. But everything depends upon your A/C draw, other rig draws and the actual voltage.

I'd be cautious about running a 15K BTU A/C on 20A.

This is going to depend on the size of the A/C unit. If your talking about the rig in your signature (which i also have)you should have no problem. I run one A/C unit when getting ready for a trip or doing maintenance at home. I actually plug into a 15 amp circuit. One of my units pulls about 12 amps. I make sure nothing else is running on that circuit when the A/C is on.

Sounds like a fair warning, though not fully explained. Depending on the draw and if other things are on a 20amp gang, you might be warned to monitor your power. Get a Kill-A-Watt or volt meter to monitor your 120 voltage. Read your AC manual. Less than 105volts on most AC will cause cumulative heat damage to your AC. This does not show right away but will take years of your AC life. I do not like going below 108 volts.