Our fresh water tank is also under the bed. We have removed the screws to lift the plywood…it's not hard if you lean the mattress against the wall or pull it out.
Per the owner's manual: " A single sensor is installed on the sidewall of each tank, near the bottom, via a 3/4" female NPT spin in thread."
There is more information about the sensors & the monitor operation on pages H-9 & H-10. But, no advice on how to get to the back of the monitor. The manual does say to contact CATCON if not working properly.
Micropulse Systems Monitor
Not sure if the above contact information is current as it's out of our 2005 Manual.