Why was the regulator replaced? Was it because of heater (furnace?) issues?
It might be possible that some contaminants entered the gas line and now the Furnace has a plugged gas orifice.
Since the gas line was open during the regulator change has the gas line to the furnace been totally purged of air?
Make sure there are no kinks in the line feeding propane to the furnace. On my TT all the gas lines to appliances leave the main line from the regulator at a manifold made from several Tee's joined together. When I took delivery of my TT one of the lines was kinked at this manifold and when disturbed closed off the rest of the way. Worth checking.
Just thoughts. First make sure there is gas to the furnace. If there is then the problem is IN the furnace.
"A wise man can change his mind. A fool never will."
"You only grow old when you run out of new things to do"
I've been having similar issues with my 2021 Grey Wolf with a suburban furnace.Similar symptoms, the fan would run for a minute or two and stop until I reset the thermostat and it would try again... The "Rv Doc" in Emporia Kansas replaced the motherboard and it ran great! For three days. Scratched my head and my wife called the dealer we bought it from and uncorked on him! After searching this forum it turns out these furnaces need quite a bit of lp pressure to run correctly. I had taken the tank off and it still was roughly 1/3 full. But I changed it with a full one opened all valves very slowly and it's been running since. Sorry for the long story...