Our Marquis has metal (I assume steel) stems bolted into/onto the wheel. If the inside-the-wheel seal of the stem is corrupted causing the leak from the stem where it's bolted onto the wheel, then a new stem is likely needed. If the leak is at the valve, then a new valve is likely needed.
If the leak is at the wheel, then I suggest first tightening the nut that holds the stem onto the wheel. It may simply have loosened up. If that doesn't fix it, then a trip to a tire shop, or O'Reillys or Autozone, to get a new stem and/or valve is easy. Someone at the shop or store should know what you need.
The problem is you may have to remove the wheel and tire from the RV to break the bead so that a new stem can be inserted from inside the wheel. Thereafter, getting the bead to reseal so it can be filled with air could be an issue. And removing the wheel's stud nuts and replacing them is another issue with which to fight. I hope it's not a leak on an inside dual tire. That brings on a whole lot of complications dealing with a stem extension.
Good luck! I would take mine to a tire shop if one is nearby... assuming you can get enough air in the tire to drive it that far.