Not sure if this will apply to your situation or not, but...
The first year I had my rig, I used to always have pressure issues. I was constantly complaining about my shower & kitchen sink. Took it to the dealer... they checked. Said no problem. Took it to another place 2 months later... same thing.
The first winter I had it, I did what I call a "partial winterization" of my rig. I drained all my tanks, including the hot water heater. Next, I opened every faucet in my rig. I had bought one of those air fittings... and with my air compressor set to 60 psi on the regulator, I blew air through all of my water lines to ensure there was nothing trapped in them that would freeze. Our temps rarely dropped below freezing where I was, so I didn't bother putting anti-freeze in.
Anyway, to the point... the next time I took my rig out the next spring, my water pressure issues were gone. I had the first good RV shower I'd ever had.
Great pressure everywhere, and I've never had any issues since.
So you may wanna try that. Blow compressed air to brought your lines. If anything is in them restricting flow, it'll probably get blown out.
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