Follow-Up:
As "Harlow" pointed out, there is a filter at on the pump (which is a standard water pump used in most RV freshwater systems) that is probably clogged. I had this problem halfway through my first shower in my Rialta, and the filter is definitely not easily serviceable because of where the pump is located. Your Rialta instruction manual outlines how to service this pump filter and where it's located in your particular floorplan. The best solution to this problem is to stop the junk from getting to the hard-to-access filter in the first place. A lot of Rialta owners have done this through a combination of methods. You can read more about it at the link below.
Here is a link that might be useful: MotorHomepage Rialta Message Board