You didn't mention what model of toilet you have... but from the Traveler/Sealand manual:
Flush valve will not open;
Possible causes:
a. Broken shaft.
b. Shaft not fully engaged in the
spring cartridge.
Service instructions:
a. Replace shaft.
b. Put pressure on shaft from under
the flush ball (pushing into spring
cartridge) until it engages. You
may have to rotate flush ball
slightly until shaft lines up with
square in spring cartridge.
Diagram
As I recall there were several RV related businesses the last time I was through there. Good luck.
Greg
'96 Holiday Rambler Endeavor DP/'03 4wd tracker
Datastorm F1/D2/7000/on SatMx5
See where I am today