The other reason water flow won't increase with the larger hose is it is still connected to the water lines in the RV, which are 3/8 - 1/2" at best. In addition, fittings, shut off valves, all restrict flow more. You could use a 3" fire hose and water flow wouldn't increase in the RV. As suggested, start at the shower valve and hose, look for blockages or mineral deposits blocking the flow. Then move on to filters and operate all shut off valves to make sure they aren't causing issues.The Oxygenics shower head is a big improvement over the standard shower head, It seems like it's more flow when really it's just more air! You didn't mention a water regulator, so I won't jump on that one.