Many have the issue Tom has brought to light and don't even know it. It's like a shock absorber, you don't notice as they slowly get worse. Often, you think it's just poor pressure at the campground. I don't know if they're adjusted wrong at the factory or the nuts get loose and the screws turn in.
This is what that valve looks like. My next project is to check mine. If you do work on it, make sure you completely remove the screws and clean the tips, they can get a calcium build up on the end. You can probably screw the screws all the way down and dislodge any calcium, but it will probably go through your shower head.