Turn it off and back on several times until it lights. May take several tries. Once it lights you should be able to hear it and like Seneca said see the flame when looking through the outside vent cover.
When the motorhome has been sitting unused and you just switch it to gas and it doesn't light it may not keep trying after a few times and never light. I always wait and listen and then verify by going outside and looking for the flame. If no go then I turn off the frig and try again until it lights.
2018 Greyhawk 29MVP
2018 Jeep 4 Door Rubicon Recon
Retired and Under New Management (See Spouse For Details)