Had similar issue until.....
I unscrewed the hose at the faucet and then noticed that there was an obstruction. Is it just the shower head? If you unscrew the hose and turn on the water, do you have the same problem?
Another consideration / guess is that if it's just the shower, perhaps there is some lime buildup inside the head. One time that was the case with a shower head I had inside my home (my other "home" and was rectified by soaking the head in a little CLR solution for about an hour.
2011 Palomino Thoroughbred Ultra Lite
2010 Ford F-150 Super Crew