I've winterized a number of RV's over the years and at one time I blew out the waterlines as well. I found that "over time" on my Komfort 5th Wheel, I would see what looked like mold in some of the clear water lines like the one for the toilet. I tried to clean it out by flushing the lines with water treatment solution ( can't think if the name) but they wouldn't come clean no matter how many times I tried cleaning them.
So when we got rid of that 5th Wheel and bought our Open Range 5th Wheel I went back to the pink RV a tie freeze that I had used on our first 5th Wheel before the Komfort.
Every spring I would check the clear section of waters lines and I didn't find ant mould looking material on the inside of those clear lines.
This will be the first winter with the 2018 NL 8-11 so the other day I got the owners manual and went inside pulled the cover off the area where the valves are and it dint take long to figure out how to winterize this rig.
We are in Nevada right now and will be heading north back to BC in a week or so and it is snowing where we live, so I will most likely be doing the winterizing of the water system on the side of the Hwy someplace where it is still warm.....
It's not hard to use the Pink Solution Antifreeze and it will only take minutes to donafter you figure it out.
Soup