On the place where the power cord for our A-Liner comes to the outside of the trailer, there is a sticker that says "This connection is for 110/125 volt AC 60HZ 30 ampere supply". There is a 3-prong adapter that we use to connect to our house current and underneath (the 3-prong pulls off) that is (we think) a 50 amp adapter. One of the campsites we want to stay at has only 50 amp service. Can we use it?

Yes. You could also use 20 amp, but then you shouldn't run your AC or your microwave if you do that. 50 amp you can run what you want.

The sticker is there to let you know that you should as a minimum use 30 amps. If you use anything lower and run your AC along with a coffee pot, hair dryer, microwave, etc. could cause a circuit breaker to trip. Plugging in to 50 amp service will allow you to run pretty much what you want to in your unit, just make sure you use the adequate extension cord. Using a lower amperage rated power cord could cause a fire.
Happy camping!!

it depends on your coach!!!

50A service is 220V/50A. The campground power goes into your breaker box and is split down, just like your home.

Adapters are just that, they adapt the 110V/30A service to match your needs or the 220V/50A service to match your needs.

The adapter that you pull off is what allows you to plug into your house's 15 amp plug. When you pull that adapter off, you are looking at your trailer's 30 amp connection. It looks like this, right? Adapter

To use a 50 amp connection, you will need a different adapter. A 50 amp receptacle has four prongs. You can buy that adapter at any RV supply store. Here's what they look like: Adapter

When you buy it, make sure you get the one with the four prongs on the male end, like in the picture. They also sell a 30-amp male to 50-amp female for use by 50-amp rigs that need to plug in to 30 amp power.

Thanks so much - we really appreciate everyone's help!