SCAD Technologies builds a reliable one. I have used them for 5 years on the plastic Holding tank and now on my water tanks.
SCAD TM series tank monitors are easy to install and use sensors that attached to the outside of a tank, which cannot foul.
scadtech.com
John,
That is good to hear you are happy with the SCAD system.
On my 315, the waste tank is plastic, and the potable water tank is plastic tank, but it appears to be in a fiberglass support tub.
The picture of my waste tank shows how small the gap is on the left hand side (aft). That might tricky getting the foil tape in there.
The semi-rigid PCB of the SeeLevel sensor may be easier to slip down into that gap and get it to stick properly.
The other option is to go with a microwave radar system like the Gobius C, but that seems much more expensive.