I have a gas furnace that we run with propane. For some reason it will not stay uo to temp, we have the thermostat set at 72 and it only reaches 66-68 then shuts off. It seems to not want to kick on as often either. We just replaced the thermostat so I know its not that. Would it have anything to do with low pressure in my propane tank?

It's unlikely you have low pressure in your propane tank.

More likely is that the furnace is shutting off due to a limit switch. It's designed to keep the furnace from getting too hot.

Check the flues, make sure they're clear. Check any filters that you see in the system. Does the fan run at normal speeds?

If it is not obvious then call for service.