Using an adapter plug does not allow for proper grounding of your appliance. That's one issue. You can use an extention cord, but, and it is a big but, you must use one that is of proper amperage rating. Check the cord you are using. Does it get warm? Hot? It is of too low amperage rating. Most extention cords of proper rating for appliances will need adapter plugs if your outlet is older. You can get the proper cord at a hardware store or industrial supply store. Well worth the little extra expense. 15' cord to run a 10,000 btu airconditioner (I think 20amp) cost about $20. It is wise to get a cord of the length you need. And if you do get longer one, Roy is very correct, do not use coiled. They should not be touching.
I live in an old house (95 years old) and I have lots of electrical issues.

I have had a little work done (adding a few outlets) and it wasn't that cost prohibitive considering the added safety and peace of mind. It is very well worth it if spending so much on a new freezer and all that meat!!
If I use the washing machine in the cellar and the microwave in the kitchen upstairs at the same time, I blow a breaker. I have to go outside (usually it happens in the rain, at night, when I'm in my pj's

)) and throw the breaker.
I love my freezer. I can take advantage of meat sales and friends with venison.
I am not an expert electician (but I do work with it) but I am an expert living in an old house and dealing with outdated stuff.