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Mine has numerous drains as well. I suspect they are for winterizing, to be sure everything drains out.

I had the original 25+ yr old 6 gal Atwood propane/engine assisted heater in my coach until I replaced it recently. However, I disconnected the engine heater some time ago for several reasons.

#1 the heater lines from engine to water heater were hoses! About 25 ft round trip. I was worried they might leak and I'd never know it till engine overheated. Ran a bypass hose and eliminated the risk. (If they were metal lines I wouldn't have been so worried.)

#2 the engine runs at about 205 F. Most home heaters are set 140-160 to avoid scalding. The RV gas heaters and electric coils run in that range, too, but if you ran hot water while or shortly after the engine was running you get REALLY HOT water.

#3 most of the time we use hot water water was not like #2 ie after the tank had cooled and the engine heat was lost. We just don't seem to need hot water right when we shut down, usually later after dinner.

#4 having just replaced the water heater in my Regal, the engine heater lines are a PITA to get to, even to disconnect as I desired (under the sink). The rest was easy from the outside... drill out rivets, disconnect gas line and wires, pull out...push in new one...

#5 the engine heat option costs more money, not worth it to me.

The old Atwood was still working but so old and crusty I knew it would die soon. It will give you hot water very quickly. The new one works great, fits right in and I used the old outside cover (with matching paint) without modification.

I have plastic plumbing and it scares me. I was thinking of PEX also. Let me know how it works.

One is for tank overflow.
One is to drain the tank.
One is to empty cold water lines that run inside the coach.
One is to empty hot water lines that run inside the coach.

We have five water drains in total
2 are overflow drains-- one for each water tank
one each for cold and hot drains
one large drain to dump the fresh water tanks

Bud