Check for voltage on the wires to the back of the GFCI receptacle. Power goes from circuit breaker in load center to GFCI. From GFCI it will power other receptacles downstream. The load and test buttons on the front of the GFCI will not latch if there is no power to it.
Lights are usually 12 VDC. Look for a wall switch. If no wall switch check for voltage at fixture. If no voltage at fixture, look for blown fuse in distribution panel. If lamps are 120VAC, fix GFCI issue first, look for wall switch, and then see if 120V is present at fixture.
I don't know if a schematic is available from the mfg. Coachmen was bought by Forest River RV in Indiana, it now operates as a division of FR.