Welcome!
I don't think I can figure this out for you, but it'd help anybody who tries, to have the answers to some questions?
What is the Make and Model of the Furnace?
Does the furnace FAN run when it won't light, or does simply nothing happen?
Is this Jayco a Coach or a Trailer?
What size Propane Tank(s)?
How full?
What's the outdoor temperature where you are?
Will it work/not work at different times of the day?
Is your coach (house) battery in good condition, fully charged?
How do your other LPG appliances, Stove, Oven, Water Heater, work?
We have a few RV techs here, and hopefully one can help you. Meanwhile, one of them, Chris Bryant, has the website
BryantRV.com with a lot of Service Manuals. You might find some useful troubleshooting there.