Total up your BTU load and then look at the attached chart.
You will see that a 30# cykinder 25% full at 40degrees will only vaporise 24,700 btu depending on furnace size, if water heater and fridge are running and you are using the stove you are real close to that.
The way to fix it is keep the tanks full and or get bigger tanks
Any propane tank at this time of year under load will show a frost line. Its normal you dont notice it in the summer.
If you are concerned about a leak mix up some soapy water and brush it on all the fitting and connections, it will bubble it there is a leak. This time of year use low temp windshield solution instead of water.
Jim & Debbie England
Do you have Gas? 2015 F350 6.2L CCLB DRW 4.30 axle. 2012 Yellowstone Ridgeline 34RLT Fifth Wheel