Anyone ever hear of flushing a dead battery with boiling water, then filling with acid?

I've read dozens of methods of restoring one but NONE have used boiling water. In fact, boiling water is a sign that you have big problems inside the battery.
There are numerous web pages telling of tricks to do it. I'd go with those from professional battery companies and ignore the DIY home brew methods.

I would buy a new battery.

I use batteries longer than some. But in some ways more conservative too. Do you want to be stuck in the middle of the woods?

If 2nd battery and you have a diode, try what you want. But neer heard of this, and have my doubts. I work with 125 VDC for a living.

If a battery is completely dead, it's for a reason. Leaving the lights on over a weekend is a reason and draining a battery all the way down will damage it. Any battery that's been drained completely down will likely be damaged and I'd rather not be dependent on one 20 miles from nowhere. Draining a good one once by leaving something on won't hurt it substantially but a number of times doing that will cut the power it can put out.

I worked in the hi voltage utility field for many years. We had 125 volt battery systems to trip the breakers if there was a power outage. when the batteries died it was pretty much over.

Cleaning it with boiling water won't cure your battery, it's dead and you need a new one. I've spent decades working on NiCad and lead acid batteries, no amount of cleaning is going to bring a lead acid battery back to life, especially without knowing the condition of the plates. Also checking the specific gravity of your battery(if it's not sealed) will tell you if it's good or not.