Interesting ideas, but at this stage I am married to the ultrasonic distance approach.
Fittings and wiring in place and all.
I have some similar Freescale pressure type sensors. They are designed for dry air, so there would be issues with that.
I am very interested in trying a load cell to monitor the level of propane in a 300 lb cylinder, but that's a project for a different day. I am curious though, if folks have real experience with a permanently installed load cell under a heavy load. Without the ability to zero it out once in a while, does it still provide usable, accurate weight readings month after month, year after year? In weather?