foggywaters wrote:
Steve. Thanks. This sounds like the Tank Tender system: www.thetanktender.com

"Tank Tender operates on air pressure. Select the tank, then pump a
small amount of air through tubing to the bottom of the tank. When the
air pressure in the tube equals the liquid pressure at the bottom of
the tank, the gauge displays this pressure in inches".

They list at US$375. for a single tank system. Have to find a non-
marine version. ?? www.grainger.com list several gages alone from
US$65. - $300.There are some lised on ebay as well -

It is an option but I only have access to a aft corner of one tank and
the fwd on the other tank - (belly tanks 168 cm L x 43 cm w X 28 cm
H). some people with the same boat have said they do not find the Tank
Tender very accurate - not that we really need precision - probably
because the tank is shallow and the tube is at the end..

foggy

If you want a low cost gauge, how about using a simple manometer and a
hand pump. Just type manometer into e bay.
No idea if this method would also work with the toilet waste tank. If
it does I would use a different tube and gauge for the black water and
drinking water tanks.............