Pressure isn't really the issue - it is flow rate that is the concern. Most built-in regulators, as well as the "on-th-hose" types have a low flow rate, usually no more than 2 gpm. Get rid of the factory regulkator and buy one designed with a higher flow rate. "Whole house" regulators usually can supply a good rate, but there area few screw-on regulators on the market that are also OK. Unfortunately I've forgotten the brand names. Suffice it to say you should not buy a regulator unless you can determine its flow rate and assure it is 3 gpm or more. A 3/8" I.D. through the regulator will assure the flow rate is as good as the plumbing in your RV.
If it is only the shower "pressure" that bothers you, consider removing or enlarging the flow restrictor that is built into the shower head. With adequare flow, pressure as low as 30 psi is probably satisfactory.