hrhgroup, i really dont know well about product you mentioned . What you have to be aware of is that company (
www.temposonic.com) is quite young (2 years) and you should get some refernce information. May be you can call them and ask for companies who bought that sensors, to get references from.
You can of course order one or few sensor and make a test in under different conditions.
If your tank is statically located - it can be possible to put loadcell under tank itself or measure weight of vertically mounted pipe
connected to tank at the bottom. Another method used is to make symmetric transmission
line in form of "U" and put it as delay element to ring generator. imho, i so somewhere in internet - the saudis did that. The time propagation and ring oscillator frequency will be dependent on amount of fuel in contact with propagation measuring transmission line. No electric contact is necessary - propagation will be depended on environment dielectric properties which is function of fuel level in tank .
Or as Archimed did ) put long closed pipe hung by wire to weight sensor into tank and measure it is weight .
Or another may be stupid idea - measure level by triangular optical method - divorced laser and camera mounted on the top of tank . Or if you can find fuel protective presure sensor - measure pressure on tank bottom .
And dont forget what the flatulent said - this could be dangerous if not done properly .