I really hate when RV mfg. install appliances so that they have to be removed in order to work on them
Unfortunately if there is no access from inside (no false floor/panel in kitchen cabinet??) then YES the whole water heater has to be pulled to gain access to electric element and associated set of t-stats. PITA
Is the spark good strong CLICK?
When main flame lights off an electrical charge is generated thru 'flame ionization'. This milivolt signal goes back to control board proving main flame which holds DC voltage on gas valve solenoids. If no signal or signal gets dropped the DC Voltage on gas valve drops out.
Dirty (carboned/sooted) electrode, poor ground connections, electrode not in main flame OR bad connections on control board will cause 'intermittent' firing issues
Other things that can affect main flame is the burner tube ......needs to be aligned so that gas flows down center of tube and flame spreader on end of tube should be parallel to tube.
Check that main orifice is clear ....remove from gas valve housing, soak i alcohol and then blow air thru it.