They do make a stick-on propane tank sensor that should show the level. It was just a very few dollars and we decided to try it. DH is less than impressed.
Our problem is that we rarely run completely out of one tank and don't think about it until we're in the middle of nowhere. Fortunately, we don't use a lot through most of the season. Right now we're at one trip this coming weekend that will require we use the furnace. Since we aren't heading toward the propane place between now and then, DH just swiped the extra one from the BBQ grill and straps it into the bed of the truck, just in case. In a normal season when we only use the furnace in early spring and fall, we can get through with just one tank.