Often that inlet on the "Service" side where you hook onto the hose has a pressure regulator. Or you may have one in the line .. Many of these restrict flow.. I will describe several pressure regulators and test results (prsonally tested)
The standard cylinderical type about an inch in diamater female hose connector one end male the other.. JUNK restrict flow
High Flow/High Capacity version of same (Wings or a bright plastic collar on the female end) Same result Junk restrict flow.
Sur-Flo (A Watts company) this is the unit built into many RV's also you can buy an "in-line" version... Better I give it a 8-9 on a scale of 10
I have used both Watts and Zurin "Whole house" these are designed to be installed with pipe and have threaded IPT fittings (or copper pipe fitings) and you use hose adapter.. Back when I last bought Around 2012 Around 60 bucks. They have a bell shaped top with an adjustmen screw/bolt where the bell would normally attach to the yoke (Or thehandle of a hand bell would attach)
Mine were 3/4 Inch models.. NO reduction inpressure from static (50 psi) to Full Flow (2GPM on the shower) When they wear out you can but a rebuild kit too.
Valterra makes an adjustable. Never tried it. Others say good thigns about it.
Finally There is a good chance that one of the square Doors (About 8" square) with a key lock... The common key that fits nearly every RV.. There's that gravity fill cap behind it.
