First off some RVs do have a built in pressure regulator or filter, but most do not. If your's has one, that could very well be the culprit, as mentioned already.
Also, as already mentioned, you could have debris or sediment build up in the fixtures themselves.
If you eliminate all of the above you will find that the sinks and the showerhead have a water flow limiter that limits the flow to from one to two gallons a minute. It is apparent when you take any of them apart.
If you remove them after checking the above mentioned items first, your pressure should increase dramatically. Along with your water use, which can be a negative if you depend on your freshwater tank and boondock a lot.
As well if you remove the water restrictors in the showerhead, you'll probably lose the anti siphon and dribble function and when you turn the shower off and then on again you may be frozen or scalded momentarily, depending.
The half inch PEX plastic tubing can deliver the same pressure as a house to your faucets, but it is wise to limit the pressure, because of the many connectors in an RV, to the 40 pounds recommended. At that pressure, unrestricted by blockages, clogged filters or pressure reducers, and with the restrictors removed, they will deliver the same kind of flow as a house does.