We have fitted a refillable gas (LPG / propane) tank to the van and it is handy to be able to tell when it is getting low so we can top it up before running out.
There is a mechanical gauge on the tank that is magnetically operated from a float inside, but it is *very* temperamental and erratic so I don’t really trust it.
This gas level gauge seems to be available under several brands – AP Products, Mopeka, and now Gas-It.
It sticks to the bottom of the gas cylinder magnetically, measures the gas level utrasonically and communicates it via bluetooth to your phone.
I ordered mine from AP Producs via Amazon in the USA as they didn’t seem to be available in the UK at the time. They now seem to be sold by Gas-It*, but are a bit more expensive than they were from Amazon.
This video (hopefully) shows how it works.
We’ve used it for over two years now, and it still seems to be working OK. The signal is strong enough to be picked up pretty much anywhere inside the van.
*I don’t have any connections with Gas-It (or Amazon…)