There are at least 2 safety, shut off on the heater that maybe giving you a problems.
1 There is a small light weight vane in the air flow path that tell the heater, the fan is working.
2 There is a temp sensor that looks for heat. There is a igniter to light the gas. you should be able to hear it, clicking.
As other have said the easy way to check the gas is light the stove.
do a search on this site on heater will not light, to get more information.
Steve and Adele
2014 Flagstaff T12RBST
2005 Toyota 4runner 4x4