I believe the stem needs to be installed from the inside of the rim, and locked in place with the nut you see on the outside. (Along with rubber seals.) The tire will need to be removed to do it. Well probably not removed, but the seal to the rim will need to be broken and resealed afterward. A tire shop project, not a DIY. Should be relatively cheap. Mobile tire repair guys can do it too, if they have the stem you need with them, not quite as cheap.
'04 Alpine Coach 36' MDDS