I'm just a guy who works at overcoming what "engineers" do to make RV life difficult. :) :) If you are happy with a 2gpm shower just use a bucket and a tea cup. That way you won't be on the forum asking how to get a decent shower in your RV. OR...you can get a variable speed pump that will keep the flow pretty constant when someone else opens a faucet while you are showering.
Engineers...the guy that makes you stand on your head to add engine oil, puts plastic coolant surge tanks in direct sunlight, mounts toilets so you can't even wipe puts windshields in to big holes...and the list just keeps on rollin'. :) :)
NOT a personal attack...just an observation based on real world experience/use in 30+ years of RVing. AND...my little brother is in the industry, we have this discussion frequently. :(