One heavy draw item at a time should be OK.
BUT Few homes actually have 20A plugs. Most likely they have a 20A circuit with multiple 15A plugs on that circuit. That means you have to share the power with whatever else is running in the house. That might be very little or it might be a significant draw.
So talk to the owner and see if you can identify a lightly loaded circuit to use and don't be surprised if you just get a blank stare.
Plus the length and size on your extension cord is a factor. The shorter and bigger the less voltage drop. Therefore buy a 10 ga 50' extension cord for under $100. Price is not a factor as you don't have CG fees.
