Is there an basic or average temperature that the warm air should be that is coming out of the heat vents? It seems that even at it's warmest the temp of the air coming from the vent is only about 74 degrees, which once it moves out of the vent a couple of feet feels cold.

If you have a heat pump it could run that cool but I believe you are having some other kind of a furnace problem. Have you had your unit serviced lately?

This is a brand new unit installed in July. It is a propane furnace

You have a problem. Temp way to low for propane if being used to heat 55 to 60 degree return air. Call the company that installed your unit. There should be a one year labor warranty from the installer/seller. If you bought it on the net no warranty for labor and good luck getting parts under warranty.

You might check your gas pressure.