It looks like there are two builds for that year and model so getting the correct drawing requires knowing if the 7th digit of the serial is 1 or 2!
And the build does change the layout of the exterior shower, so perhaps we can go with letting you do a check of the interactive drawings here:
That takes a bit of practice to get good at turning it all over and around to spot the small details but maybe you can spot things which I would miss when not knowing your coach?
One thing that I would want to check is if there are other plumbing items which also have problems as a way to cut down the possible causes???
However, it would be a first thought that there is a low point drain along that line which is left open, so that pressure can't build on it. Possible you have been draining, getting ready for winter and missed closing one?
Looking at one of the two builds, I spotted this as a suspect----but that depends on guessing which build, so do check that before trusting!!