my thoughts too, with the weight of the system and the tanks, 1 litre of water weighs 1 kg, it would mount up. so for a trip out, your would need to rinse after launching, and rinse after retrieval. if you can get it down to a jerrycan for each, factor 25 kg for each jerrycan. provided you do not alter the structure of the trailer, by ubolting and hoseclipping everything in place, it should pass test. it would be worthwhile getting your trailer and fully loaded boat weighed first so you know how much additional weight you can add (the boat is supposed to be empty whilst on the road, not full of gear, but allow for that anyway, and watch your towball weight. this is for cleaning the trailer and possibly the engine right? the boat structure gets washed later at home, as it is better protected by paint? I considered spray coating the inside of the trailer frame with tectyl, making my own super long pressure sprayer