water heater plastic plug removal

Some people install water accumulators in RV to lower pump fluctuation and also to have some pressure for midnight pee without waking up family with rattling pump.
I have the same benefit without the accumulator.
I think the primary function of air bubble in small rv system is to lower pressure build up when the burner comes on. Otherwise you will see PRV dripping during warm-up period.
Not that building system pressure to 150 psi is a good thing.

The PRV is located on the outside on all water heaters is standard safety with the same as a house HWH, it is the same PRV. Your saying because of the outlet being lower than the top there is an air bubble that in essence can never be purged. You do have to purge up to that point though otherwise you can crack the tank if turning on prematurely. The air bubble you speak of does not negate the need for the pressure relief valve. That is all I was trying to say.

Sorry for the confusion.... I should have been more clear on that.

Again, that applies to RV WH as household WH have different principals.
As you can see on the pictures, the hot water outlet on RV heater is placed some distance from the top, what keeps air bubble.

I don't understand what you are doing for an air bubble nor why doing such a thing is needed, I have been around great plumbers for many years and always do as the plumbers do and that is to turn on the water pressure with the hot side open on the faucet till all the air is purged out and the water flows solid. Then you can turn on the water heater if needed. you want no air in the HWH as the pressure relief valve takes care of the safety aspect. They are the same PRV that is on house HWH'rs as well. I wonder sometimes why I even try educating people on these forums as it seems opinions are becoming solid knowledge now a days

Each WH has safety valve. I think those in RV are only pressure valves, when household WH has pressure/temp safety valve.
Still I would not count on plastic plug design to work with 150 psi pressure to be much of safety relieve.
BTW WH in RV are design to have air bubble on the top not only for safety but also for convenience. The air with time gets sucked by water what you can notice by pump making big fluctuation with drizzling faucet.
How many ever tried to restore the air bubble?
For me it becomes routine that when I hook up empty hose to my camper, I open hot water faucet so the air from the hose runs via WH restoring the air bubble.

HMS Beagle
That is what I was told many years ago along with acting like a freeze plug.

I was under the impression that the plastic plug is a safety device - intended to soften and/or blow out in the event of overtemp/overpressure. Replacing it with a metal one will defeat that purpose.

In my home repair business I made quite nice chunk of money replacing galvanized nipples put into copper plumbing.
For housing developer saving $2 on a nipple multiply by 12 nipples in the house, multiplied by 200 houses made it worth the risk - especially that the electrocorrosion acts slowly and usually the nipple start spraying water after 15 years.
But it is unavoidable.

I have a brass plug that has a built in petcock to drain.
bumpy

Once you get the plastic plug removed, eliminate the problem with a trip to hardware store
Add a ball valve to drain hot water tank. Used this setup on last two campers with no issues.