That both the water heater and furnace won't run does sound like a propane problem and not the appliances. As Dan suggested, check the tanks to see if they are full (or at least not light). If they have been working and this is a recent phenomenon, it is possible that you've been visited by the tank auto-switchover valve gremlins. When this happens, if you have the auto-switch set to one tank and it runs out, the flow changes to the other tank. When the valve indicator fails to change to red on the first tank, you think it still have fuel, but are actually now emptying the second tank. Eventually, you wake up in the cold (BTDT).
The other mistake is to have the selector set in the middle, so it empties both tanks at the same time.
It's not cheap, but investing in the Mopeka tank sensor system (with smartphone readout) is one way to keep an eye on your propane supply without having to heft tanks or rely on the indicator.