My SMB has a mechanical gauge on the tank itself. The VW camper I had before didn't, but I never used the propane on that one enough for it to matter.
With BBQ bottles I usually estimate the propane level by looking at where the condensation is on the tank, or feeling it with my hand to see where it gets cold. That's the level of the liquid propane inside. Only works when you're actively using propane, though, and probably wouldn't work as well on a permanent tank due to the thicker walls.
1990 E-250 Sportsmobile w/ penthouse top, converted when new by SMB Texas.