I’m probably beating this to death, but… (haha)
if you happen to have an outside outlet on your house that’s within 50 ft of your garage interior, you could use a 50 ft 12-gauge extension cord. I just looked at my 50 ft 12-ga cord and it’s labeled for 15 amps, which should be sufficient for a 5000 btu a/c. chances are a 100ft 12-ga cord would also be labeled for 15 amps but I don’t have one here to check.