Water heater or plumbing?

I have been experiencing a drop in hot water pressure from all faucets and showerheads in my home. I drained the water heater (got sediment out). The pressure improved for about a week, but has decreased ever since. The water heater company says it's the plumbing, the plumber says it's the heater (may have been mishandled before installation). Water pressure outside is 45 psi, coming directly out of the heater it's 20 psi. Heater company says water heater doesn't affect pressure, try flushing the hot water line. Plumber mentioned the galvanized pipes directly meet the copper ones, but doesn't think that is the current cause (though I do need to have that corrected). But only part of the house has vertical galvanized pipes, the rest has copper, and this pressure problem is from every hot water outlet. Cold water pressure is fine. Plumber did not report any leaks. Gas heater, 18 mos old, hard water. When they're pointing at each other, who to believe? Any other possible causes? Thx