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Yes. All kinds of excuses but it is what it is. So, doing a lot of work de-rusting my 710. It's been a couple months since it's been started etc. I've had the batteries on trickle charge and are healthy. Putting stuff back together and briefly connected a swapped neg/positive position. Now I have nothing, no clicking, no lights or lights on breakers etc. Battery switch is in correct position. I tested both batteries and they are reading healthy alone and 24 plus volts together, but one of them shows continuity between posts. Is that a short? I sure could use a few bread crumbs to proceed. I feel stupid and I'm angry with myself for not paying attention during this phase. I've put a lot of work in this thing and I think I bricked it......

If I understand the "Oops" you connected the Batteries as: (-B1+ to +B2- )

In that case, the effective output voltage would essentially be the difference between
the two batteries and either slightly negative or positive -- assuming these two batteries
are approximately equal.

OK, well what could happen. If the difference where sufficient to activate the starter
solenoid and you pressed the starter button, then the starter would create a path
through these batteries and the reversed one could be damaged. But for that
scenario to happen the difference would have to be at least 6V or more to activate
the solenoid -- which is not likely with two good batteries. You may not have even
touched the starter button for all I know

So...you may not have done any harm...and you just might have a bad connection to
Ground through the battery switch or along the positive lead. You touched a boat load
of things and something just might be loose.

Can you elaborate on what you mean by "continuity between posts" ? Weather these
are wet cell or AGM, they will be very low internal resistance. If there's an internal
problem you will see that as a discharge even warm to the touch.

I can't thank you enough for replying so quick. I really appreciate anyone willing to help debug situations with these vehicles. So, in my haste to hook everything up after being down for a few months, I failed to hook up the master battery switch ground. That probably saved me from any serious issues hooking up the battery incorrectly. It's running like a champ now. I'm going to take a break from this cool truck as I've been working on it so much my life balance is out of whack. Thanks again.....