If your brightness adjustment dies not bring the lighting up to an acceptable level, you are going to have to dive in behind the dash with a test light and a volt meter.
There is a knob to adjust the brightness of the instrument panel. I don't know if it only affects the brightness when the headlights are on or if it can also be used during the day when the headlights are not switched on. The Ford chassis owner's manual has some information ("Instrument Lighting Dimmer"); if for some reason you didn't get a copy with the RV, you can find one online at the Ford owner's web site.
