Low water pressure within your RV is usually caused by clogged filters or aerator screens. How is pressure using the pump? If it's fine on the internal pump, but slow on city water, it could be too small a hose, a kink, or a valve not completely open if you have to change it when going from tank fill to city setting. A filter can impede flow even if it's brand new, especially if water pressure is marginal. The way to test would be to remove the filter cartridge and put the filter holder back on. Taking shower sprayer off and soaking in vinegar can clear up deposits in the holes. Need more details for more suggestions.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI