what gauge extension cord should I use for a 19 ft. travel trailer with an air conditioner using 110?

Well there must be something else in there (fridge, lights, etc..) I would not use anything less then an extension cord with a grounded plug and 12 gauge wires.

.

How many amps and how long do you want the cord to be? Check the following pages. If you are unsure, go with 16 gauge. Make sure it is UL listed and has a circuit breaker end if the A.C. doesn't already have one on it's plug. Make sure it is meant to be used outdoors if you think water might be an issue.

Outdoor rated cord with 12 or 14 gauge wire.