We are contemplating undertaking the same project on our motorhome. I made valences for a TT we had prior to owning our motorhome. I built the valences from luan and covered them with fabric. sc3283 is right - it is kind of like wrapping a package.
Remove one of your valences and check it out to see how it is covered. The hardest part in recovering it will be pulling out all the staples they used. Put on the new fabric in a similar manner. Fold edges under, tuck in corners and staple. The fabric usually has a little padding under it, and you can buy batting that comes in thin layers at most fabric stores if you can't reuse what is already there. Buy extra material in case you make a mistake.
PS: you can also use fuseable interfacing to help seal edges where you can't get staples in. Fuseable interfacing is a non-woven fabric coated with adhesive than you can use to join fabric together or stick the fabric to the wood backing with the heat of an iron.
2015 DRV Tradition 375KPS
2015 Chevy Duramax 3500HD LTZ Dually
Full-timers...Home is where we park it.
Check out our blog: Living Our Dream