The reversing sensors on my Octavia were always playing up but would often heal up after the car was cleaned with a jet washer, individually they all tested OK, they have been working fine for a long time now but I think you have hit on something.

 

I too have a universal towing electric relay and as it added to the standby power consumption I removed its fuse to disable it & refot it when towing which I havn't done since, that corresponds with the period that the sensors have been behaving.

 

And then it dawned on me that when  got the car I disconnected a ring terminal from the battery not knowing what it was, a long while later I found the trailer lights were not working, it was the power lead to the trailer wiring relay and I reconnected it to a terminal on one of the battery box fuses, that was the time that my reversing sensors started playing up!!!!!!!!!!!!!!!!!!!!!!!

 

So my advice to you is to try powering down the trailer lighting universal relay to see if your reversing sensors then work, it will do you no harm at all to fit an isolator switch like I have done in the Yeti or simply remove the fuse when not towing, not only might you fix your reversing sensor problem but it will save your battery from being killed off if you leave your car unused for an extended period like a holiday or the next confinement.

Which fuse are you removing? 

 

Thanks

Oly