Is this problem when your hooked up to shore power or just with Battery
If the battery
is low it will not have enough power to open the gas valve.
Can you hear the igniter clicking or see the spark when it is trying to lite?
If the answer is yes and your sure you have a good power (mine needs 12.4 volts min. in the battery
to turn on gas valve.) could be sever other things.
1. Igniter is not close enough to where gas comes into burner chamber. This can be dangerous because gas can be building up in the chamber then Boom....
Can you smell gas???? If so and it is sparking just bend the igniter so it in the path of the gas tube.
2. the Fan is not turning fast enough to make the safty switch in the back of the furnace
connect with will not allow the gas valve to turn on.
Try to locate wire and only if sure you've found the right ones you can jump them.
3. The gas valve is junk.
I've had all these problems over the years and all but #3 is a cheap fix.