Good to know Year/Make/Model of the Coach, but we also need same for the Chassis. Chassis may be a model year (and usually no more than one model year) earlier than the Coach was sold and titled as.
Do your bulbs look like this and push into the sockets?
Or like this and push/twist in?
Never mind they're Amber, just the first pix I found... Note that they only seat in the sockets one way. What I really want to know about the sockets: Is there enough metal near/part of the Socket that you can connect a temporary jumper to a known solid ground, like trailer hitch, to make sure original ground hasn't failed?
Also, CHASSIS builder made up the FRONT light circuits but COACH builder (sometimes called UpFitter) connected the REAR when they installed the House onto the Chassis. Could be a Splice or Connector. Some Chassis (at least Ford I'm familiar with) have separate fuses Front and Rear, and some have more, called Trailer. The House could have been connected as Trailer.
Are the Marker and License Tag Lights coming on with Park/Running Lights? You said "tail and backup" were OK but look up at the Markers too.
Got Owner's Manual? You may have missed a blown or loose fuse.
Done/Plugged anything electrical beside change bulbs?
If God's Your Co-Pilot Move Over, jd
2003 Jayco Escapade 31A on 2002 Ford E450 V10 4R100 218" WB