Suburban water heater

I'm wondering if you have pure propane or perhaps a mixture of propane and butane. Pure propane will not exhibit the problem your experiencing. Freezing does affect the flow of butane. I ran into this problem a few years ago around Thanksgiving time when visiting our daughter in Michigan just North of Detroit. It got down to about 15-20 degrees F. Tank would get down to about 1/4 full then the furnace would shut off. Confirmed I had a mixture of propane and butane from folks who were filling my tanks. Went to a different location to fill the tanks with pure propane and problem was solved.