Yeah, we have Meenan for our service also, but since replacement parts are no longer available for our unit, they said we'd have to replace the whole thing, not just the coil. The service tech did say that we could install a separate electric water heater. Right now that's just not feasable for us, $ is really tight right now, and we would have to get a plumber to install the pipes and whatnot. The whole problem is just really weird b/c it only presents itself in the shower, I ran the hot water in the kitchen sink for 10 minutes straight the other day and the temp didn't drop anything like what it does in the shower.
I tried to send a pm, but the app said permission was denied.
The difference is your shower is probably out putting 2.5 Gallons and your faucet is only something like 1.5 maybe. I put a lower pressure shower head which gave me a few extra minutes at times. It was a cheap$2.50 one from Lowes that is meant for an RV.