Try here for a Ford owner guide :
The dual wheels may be impossible to check the air in. They must be installed correctly so you can access the inside valve stem. On some Ford wheels, there is only way this is possible. Most Techs don't have a clue. Good luck, and don't depend on the factory jack picking up the rear of your RV. Go buy a nice hydraulic bottle jack rated at more weight than 1/2 your gross weight, then you won't have any problems. Sell the factory jack on EBAY.
2012 Ford F250 ext. Cab 4x4
2002 Jeep Wrangler Sahara
2008 Yamaha V-Star 650 Classic
2008 Work and Play 18LT