Buy one of these blow out fitting at your local RV shop or on-line.
It screws onto the city water connection on your trailer. Attach an air line to the fitting. Keep the air pressure from your compressor fairly low ~30lb so as not to damage any of the plumbing. Go through the trailer opening each faucet and you'll see some water come out then just hissing air. Close that faucet and go to the next one. I'd suggest starting at the closest faucet to the city water inlet moving towards the one farthest away.
And don't forget to drain your hot water heater tank. Many have an anode rod attached to the drain so it also gives you the chance to inspect it while draining the tank.
Barry, MSgt, USAF, Ret. & Rose
2015 Nissan Frontier V6
2014 Coachmen Viking 17FQ