On my RV, I started using the cheap, non-adjustable water pressure regulator. When I changed to an adjustable pressure regulator, I set the PSI at 45 and flow improved in the RV. They run about $50 at RV stores. Next thing to check is your water filter. Some can restrict flow quite a bit. I use a water filter designed for home use and not for an RV -- gives me more gallons/minute. Final thing to check is for sand or obstructions in your lines. You have a filter at your toilet and most likely each faucet has a screen too that can clog if you've had hard water or sand build up.
My opinion is the size of your water lines is not the problem.
Cedar Creek Silverback
Mazda Miata TOAD for sex appeal