The heater is probably automatic ignition and has a problem doing so. This is usually due to low voltage or debris in the combustion chamber. Make sure your battery voltage is at least 12 vdc and that you have the gas turned on. You my have to try turning it on and off a few time to purge the lp gas lines. If, after, trying several times with no success the unit will need to be pulled out and cleaned.
Mike, RVIA & RVSA Certified Master RV Technician
Amy, Dr. Assistant - Roxie & Mei Ling, four legs each
2000 Gulf Stream Scenic Cruiser 450 hp & 1330# torque
06 Saturn Vue, 06 Chevy Z71 4x4 & 2014 Corvette Z51 M7