don't forget to check the battery charge level, a low battery won't provide enough power to turn the fan fast enough to open the sail switch. Or it could be construction debris/lint/dog hair caught in the fan. Then there's the propane regulator as well as the ducting being clogged and a long list of other possibilities...
Don't start buying $100 parts to throw at it, hoping the first one or two might fix it.... Download the troubleshooting guide and follow through with the "logic tree troubleshooting guide"....
https://manuals.heartlandowners.org/...11-05-2015.pdf