Knowing some of the physics behind how the gas pressure in a propane cylinder develops, I just can't see how these gauges could provide you with anything more than a general idea of when you are about to (or have) run out of propane...
The physics I suspect Peter is referring to is that as long as there is liquid propane left in the tank, the pressure inside the tank is only a function of temperature. Once the liquid is gone, the pressure will decrease rapidly as the remaining propane vapor is depleted. I have one of these gauges on my BBQ grill and the gauge doesn't move until just before the tank is empty.
Also, be careful how much weight
you hang from those plastic propane connectors. They've been known to break off due to fatigue as you're going down the road. I had converted my camper over to the convenient plastic knobs used with the new-style tanks, then converted the camper right back to the old-fashioned wrench-tightened brass fittings after a fellow Scamper told me about the problems he had with the plastic knobs breaking on the highway.