Safe to use a 30 amp Extension on 50 amp trailer at home??

The real questions are, how much current (amps) does the A/C and refrigerator draw, what is the total distance and what gauge is the thinnest cord (weakest link in the chain).

Without answering any of those questions you can get a "go/no go" answer by simply checking the voltage at an outlet in your RV. If it is above 110VAC with the A/C and refrigerator running, you are good Most meters will not read an minimum voltage, but when the A/C is starting, you want it above 100VAC.