Some systems have a Normal/Fill selector (two-position valve where water gets connected). If you fill then remove hose, you have to select Normal otherwise the pump will just run without being able to build system pressure.
The other possibility is that your faucet spout and or shower head is completely blocked by crud. (I know this one because it happened to us.) Remove and clean the little screeny thing (me and my tech terms).
2010 Winnebago Aspect 28B