This one has me stumped. When shower is running and temperature is stable. If you operate the toilet the shower gets colder. I don't get how asking for cold water at the toilet would make the shower get colder. I tried the bathroom sink and it had no effect on the shower. Just the toilet. Thoughts on this.
Robert scratching head.
When cold water flows I reduces the pressure in the hot water lines.
Check air the pressure in the water system pressure, it should be about 40 lbs.
If water squirts out of the pressure tanks air valve you need a new tank.
I have heard of people plumbing the toilet to hot water in a residential home to prevent tank sweating. I realize there's no tank in an RV toilet but maybe it was done for the bowl or maybe just a mistake at the factory? You might check.
I don't think that's the problem. The water system on this coach is zoned thru a distribution manifold. There is only one water line to the toilet and it's cold water.
But that doesn't explain why only the toilet makes the shower get colder but the bathroom sink doesn't.
Other than using more water when you flush vs. sink? I do know that when I was about to take a shower, I always told wife not to do dishes, etc. Water remained within tolerable temps. Same with a residential home! No secrets!
1/2" incoming pipe is only rated for two fixtures .
You problem is generally a pressure issue, try hooking to city water and if the problem disappears then you know it's a pump/ pressure issue