I ran my 30 amp TT from a 20 amp plug out of my garage for 8 years via one of those orange 100 foot extension cords, purchased from Wall Mart. I have no clue what rating was on it.
I could run the air conditioner in the camper with very little problem. Never blew a breaker, and of course, there was absolutely no other draw on that circuit either. As long as nothing else was turned on in the camper, the air conditioner performed just fine! But if I attempted to run anything else, the orange cord would begin to get warm. And the connections would warm up too.
So, we could run the air conditioner, get the camper cooled down. Then at evening shut it off, turn on the televison and sleep in the camper all summer. We did this for ... what? 8 years.
When we got our Outback, I installed a proper 30 amp RV outlet from the garage. Last Summer, I installed another 30 amp proper RV outlet on the other side of the drive way. (completely separate circuit). Putting in the dedicated 30 amp RV boxex, we can run anything now.
In your case, the most that will happen is, your wires will begin to warm up. If they do, you're drawing too much. Simple answer? Get a heaver extension cord.
Worst case: You pop the breaker in your house! Solution? Turn something off, you've exceeded 20 amps.
Meanwhile, begin planning on installing a true 30 amp service on your house / garage dedicated for your camper and use a proper 30 amp extension cord designed for your camper. The connect box should run about $125 ... give or take a little ... Depending upon how far the wire has to run, plan a couple hundred dollars for wire (and it will probably be less). Run the wire yourself to curtail the cost of an electrician. Call an electrician, have them hook up both ends of the wire, and it will probably be less than a couple hundred dollars for their service. (depending upon where you live).
Then you can use your camper just like at a camp ground and won't have to always question your power usage.
