In this episode it's implied that Frank and Estelle have quite a bit of money at their disposal. George says his parents' living expenses are "nothing". As far as I know the cost living in NYC, at least in Manhattan, is considerably higher than the national average. Frank and Estelle live(d) in Queens. It's been mentioned that Frank was once a salesman who traveled to Korea. Maybe he had some sort of pension and/or he and Estelle had substantial social security payments that far exceeded their living expenses.
Furthermore, I know several older couples who winter in the North and summer in the South. If they've got the money, why couldn't Frank & Estelle keep their house in Queens and have a condo in Florida? winters in Florida.