I measured the a.c. outlet to my laundry washing machine at 122v. But the circuit breaker that shuts off power to just that outlet and marked 20 amp. Single breaker. How many amps is coming out of that a.c. outlet?

 

In the photo of the circuit breaker I switched that 30 amp breaker off.

My plan is to power my 15000 btu air conditioner in my new 2017 24' Outdoor RV Creekside 20FQ travel trailer while parked in my drive way for a few hours. I have a 30 amp input on the trailer with a 30amp connector doggone to 20 amp power cable connector

 

Zero amps are coming out that AC outlet until you plug something in. American standards say the load shall be designed for not more than 80% of the circuit breaker rating. 20 amps times 80% is a CAPABILITY of 16 amps. They won't come out until you plug something in.

 

Almost all electrical device's now-day's have a "DATA PLATE" which states the operating voltage and "Wattage" (sometimes the current load "AMP'S" ) required to run the device ("FL AMP'S" or "Full Load Amp's" ) check for this and compare it to what you have available in the breaker rating and wiring rating.
A 30 Amp circuit Breaker should be feeding a circuit with #10 wire (no less) or better depending on the length of the wire run .
Bottom line is: IF you are not comfortable with house wiring (and it appears that you are not) ask a locale "ELECTRICIAN" to check out the circuit you intend to use (it could save you a fire).
ALSO: Keep in mind in this case, if you are plugging in the the travel trailer , you are also going to be plugging in it's on-board converter / charger which will add to the total load (number of Amp's ) it will try and draw from the circuit breaker and wiring.

 

120v 15,000 BTU winfow unit should draw 15A or less, so it should be safe to plug in the A.C. (and ONLY the AC) to your dryer outlet. That means don't run from your dryer outlet to the travel trainer's main 30A plug; run straight to your A.C. cord inside. Make sure you use a decent extension cord.