Just as a sidebar to this subject, if your looking for a better shower in your RV, you might consider adding an OXYGENICS SHOWER HEAD. I heard great things about it and, although skeptical, I sprang for one. Best money on an accessory that I'd spent in awhile.
OXYGENICS RV SHOWER HEADUsing our stock on-board pump always resulted in wimpy showers. Adding this shower head greatly improved the shower pressure and now made it more like "at home" and enjoyable to shower in the RV, even when using the on board pump. I was surprised that something worked as advertised, but it does. Now, I (and more importantly, DW) don't mind having to shower using the on-board pump.
I changed my on board pump a couple of years ago, so it's fairly new (Standard Shurflo replacement pump). In the meantime, I bought a new Shurflo pump that is rated at 55 psi. (I don't remember the model off hand). Since the current pump is fairly new and the addition of the new shower head greatly improved the shower, I'll just hang on to the new Shurflo until the current pump gives out.
If you're trying to improve your shower in your rig, rather then throw down some big bucks on a more powerful pump, I recommend trying one of these first. Camping World sells these as well.