Does your water system work fine when you are hooked up to city water where the RV WATER PUMP is not required... The City Water determines the water pressure in this case...
If you are only having water pressure problems when using the fresh water tank being pumped out by the RV WATER PUMP. Several things can be involved here lowering your water pressure...
One quick test you can do is stick your winterizing tube down in a gallon bucket of water and operate the valves to winterizing. With the RV Pump ON you should see great water pressure coming into the sinks, shower heads, etc with the pump running...
Air leaks in the lines are one cause for low pressure...
Roy Ken