PUT YOUR WATER HEATER IN BY-PASS MODE, then remove the anode and flush the tank again. Since you start out with pressure and quickly lose it, it is possible that the "crud" is still in the heater and sucking up against the hot (discharge) line due to the outflow of water.
This might or might not solve the problem, but I think I would try it this way before I attempted to back-flush the lines from a faucet inside the trailer.
Can't hurt to try! Good luck.
Mike and Yvonne
and Sophie, the little white dog
2017 Columbus 320RSC
2015 Chevy Silverado 2500HD Duramax 4X4
“It's not how old you are, it's how you are old.”
― Jules Renard
"It's not the years...it's the mileage." - Indiana Jones