Replacing ancient Suburban water heater with new?

More then likely that 'calcified stuff' is the remains of the anode rod.
Anode rod material is wrapped around a steel rod that is welded to drain plug hex head.
Not the first time they have separated when trying to remove a drain plug

BUT it is a 43 year old water heater.
Time/effort not worth it.

That G602 should have a installation opening 12 3/4" H x 12 3/4" W x 20 3/4" D

New Suburban 6 gallon water heater features 12000 BTU's burner,
and is 12-11/16" H x 12-11/16" W x 19-3/16" D.
(All same size...gas only or gas/electric)

Looks like a fit.

(Atwood is completely different so no swapping brands w/o modifications----6 gallon 12 1/2" H x 16" W x 18" D)

Be aware of the fit and the location of connections LP and electric or you will have a bigger job replacing it.

YEP. Replacing Suburban (or its predecessors) with Atwood is difficult sine Suburban's oblong tank's long axis is crosswise in the coach, and Atwood's is lenthwise, so you'd have to change the square Suburban cutout in the wall to make it rectangular for Atwood. Let alone actually have space inside for the different layout. Atwood to Suburban would require reducing the size of the hole, and maybe not having the depth inside for the 90* rotation of the tank axis.

I'd like to hope that the LPG connections are similar, pre-Suburban to current Suburban. Probably wishful thinking...

I knew of the style differences between brands, and expected to replace it with a new Suburban (bonus - they're cheaper anyways!). Extending wiring and gas lines are simple if need be, especially compared to changing the opening size.

I did take a closer look at it yesterday, and was able to easily punch a small screwdriver through the crud, which let all the water drain out. I also noticed that it didn't actually break the drain tube, but did mangle the first thread or two so that I couldn't just screw a new fitting on. The tube appears to have a pretty thick wall compared to typical plumbing pipe, and I'm thinking I may be able to run a pipe tap into it and just use a NPT plug to seal it off if I can't get the threads cleaned up on the outside.

It's definitely not an anode, or the remnants of one. The drain valve on it has a rubber disc that's connected to a screw on the inside, with a hole drilled on the side of the valve. The valve screws on to the end of the drain tube, then the inner screw is run in to press the rubber disc up against the end of the drain tube. Run the screw back out, the disc retracts, and the tank water drains out of the little hole.

I looked at new anodes at the RV parts store, and there's no good way that I can see to retrofit one either.