I'm not certain, as I've not actually checked mine, but there may be an access panel in the back of the HydroHot/water tank bay. You'd have to get the coach raised and safely blocked or up on ramps so you can get under it and look for a removable panel into that bay. A sensor would have to be along a bottom edge of the tank to sense the weight of water above it.
That said, I can see between the left bay wall and the tank and there are wires heading to the lower tank back from above the wall. My storage bay right side wall adjoins, but visibility is blocked by structure I've added to the wall 14 years ago. Before crawling under the rig, check the storage bay's right (front-facing) wall for a removable carpeted panel that might give you access from that bay into the back area of the water tank. I keep finding access panels on our coach that are well-disguised by carpet liner or other means, so it's worth a try.
Joel