Some other quick ideas.
1) DC power is required for propane to heat water. Check battery isolation switch. Check fuse. If you have a meter check for 12 VDC power at the unit.
2) Check propane level. Water heater can draw a lot of propane and if the valves were opened too fast; the crash protection valve in the tank can close. This valve lets gas leak out but at a very low pressure; not enough to fire the heater.
3) Press the reset button on the High/low limit switches.
See attached manuals (not sure which one you need, but the photos in the manuals should held determine with one you have).
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW