If you do not have valve extensions on your valve stems, then the problem is most likely with that valve stem itself, only one working part and that is the Schrader valve inside the stem.
All the other tires take air ok?
It easily replaceable, go to an auto parts store and purchase the removal tool, unscrew the valve and pull it out then just match up a new valve at the auto parts store. There are some different sizes that's why you need to remove it first
Or just drive the RV to a tire store and have them replace it. It's quick and simple.