First, on the extension cord, when I used one (current trailer reaches the plug in the garage) I used a 'construction grade' extension cord the guys who built my wife's house left behind and never picked up for 10 years. It's basically the same size as the 30 amp cord that came with the TT, but had 15 amp plugs. (I should actually check it's gauge one of these days).
Secondly on the AC on 15 amp, with the TT, when we first got it, I once flipped on the AC with the TT plugged into a 15 amp in the garage, trying to cool it down so we could pack it without being in the sweltering heat. We then left for a few hours, when I got back, the breaker in the house had tripped. That's when I remembered that the dealer mentioned not to try running the AC on a regular household plug. Never ran the AC at home again.
When we got the fiver, I installed a hardwired EMS instead of the portable one I used to use on the TT. With the hardwired, I realized that the AC is actually only using about 9 amps. So, I figured that I could run the AC at home, as long as it was the only thing I ran. So, I've done that a few times. Go in the rig, kill all the breakers except the AC, then run the AC and it's worked fine. They key though is killing the breakers to everything else. You don't want to have the converter or the electric side of the water heater kicking on while the AC is running, as you'll go over 15 and blow the breaker.
There's no use crying over spilt milk... unless it's on your keyboard.