If there is no voltage TO the GFCI, then the GFCI will not work.
Now: The suspect list:
No power to coach, or to that leg of coach
Tripped breaker (you said you checked but did you do it visually or did you use the tactile test (Try and reset it) Some breakers do not clearly show "Tripped" all the time, but they usually FEEL tripped).
Screw loose.
Bad wire.
I am fond of saying many RVers have a few screws loose.. One of the circuit breakers in my RV took over two full turns before I felt any resistance it was that loose... In addition I had a loose screw on my power inlet.. On two other RV's I've found loose screws on the power cord outlet end (on one all the screws in the power cord were loose, all six of 'em, the end came off in my hand).
Finally.. I have had one case (mine) of bad wire.. This is very very very rare.
(NOTE: unless the wire runs straight from the breaker to the GFCI... there may be a bad connection at any point where there is a connection.