Aqua roll remote water level indicator

Its a cold dark raining night or early morning and you want to take a shower but do you have enough water left?

Nothing worse when you rinse shampoo off and run dry. (we have all done it)

Has anyone invented / or adapted an Aqua Roll so that you can see how much water is left in it from inside your caravan (like an onboard caravan / Motorhome water tank)

(I guess it would need to be robust with all the rocking and rolling back and forth to the mains tap)

Apart from remote digital (bathroom type) scales to stand it on thats about the only solution I have thought of.

I bet someone surely has done this in the CC?