If the furnace is actually igniting then everything before that in the pre-ignition sequence is fine, including the sail switch. The issue is likely with the igniter/sensor, manifold, or propane regulator.
Igniter/Flame Sensor - In many cases the igniter is also the flame sensor. If it is unable to read that the flame is lit then it will shut off the furnace. The flame sensor can either be faulty or not at the correct specified distance from the burner.
Manifold - This is the more expensive item to replace but if it isn't allowing the correct flow of propane it might be faulty.
Regulator - Your propane tank will typically be pressurized at 200 - 250 psi. The regulator will reduce that to a system operating pressure of 11 column inches of flow, which is approximately a 1/3 psi. If the regulator is not correctly adjusted or is faulty then the furnace ignition can fail to be sustained. It is possible to test the propane flow and adjust the regulator but that requires special equipment.
The easiest thing to check is the igniter/sensor for it's distance to the burner. You should be able to find that distance in the furnace documentation that came with your trailer. I saw in a video that it should be about 1/8". However, my docs say that mine should be more like 3/8". After that you might need to connect with a service tech to troubleshoot the rest.
My furnace is an Atwood, though I think many of the basic principles are the same for Suburban.
Here is a helpful video
Jeff & Sandi (and Teddy - 7lb Schnorkie)
2018 Montana HC 305RL / Andersen Ultimate Hitch on B&W Ball
2012 RAM 3500 Big Horn Crew Cab 4x4 SRW / AMP Power Running Boards / Air Bags