2014 Regal 35 Sport Coupe Level Indicators

When we first got our new to us 35 SC, the level gauges for the fresh water and waste water were not working.  The fuel gauges showed 3/4 tank on each and appeared to work properly.  After our first fueling, the gauges went opposite.  One empty and one full.  They have been that way ever since.  When you burn fuel, one fills up and one goes empty.  I decided to tackle the easy one first, so I disconnected the sending unit on the fresh water tank, tied the wires together and the gauge went to full.  I pulled the sending unit and was surprised there was no stem on it with a float.  I went to the local Regal dealer and he said it was a sonar gauge.  He didn't know if there was a way to test it or not.  Does anyone here know if there is a way to test the sonar sending unit?  After this one I intend to go to the waste tank level sender.  I'm really not even sure where it is exactly or how to access it.  Any advice here?  After that, I am heading to the port side fuel tank.  I know how to access it under the reclined seating where the cockpit H/A is located.  I looked at the starboard side and assume you access it from the bilge area and under the flooring??  All help appreciated.

Thanks,
Jim