I don’t have an answer for you, but it probably has to do the with temperature of the water coming into the trailer. This has been a hot dry summer and in some areas the ground water is coming in at 80 degrees. After passing through the tankless water heater, water that warm is probably coming out of the faucets at 115 degrees even at the low setting.
Also at this time of year a water hose will heat the water to bath temperature, even hotter if the hose is setting in direct sunlight. If this is the case you might have to turn on the cold faucet first to clear the hot water out of the hose.
Just some thoughts to consider.
Bob and Joyce
2013 CC Silverback 29RL
2010 Ford F250 XL Crew Cab 6.4 liter diesel
ATU Local 788