"Now given that spec grav is so low...would the addition of electrolyte be advised at this time.?"
Never!
Only 3 reasons it should be low, Batteries need charged or they are sulphated or you had a spill. We can probably rule out the spill, you'd know if you turned one on it's side in your truck right. I did that once in a van, I knew! That would be the only time acid should be added, after a leak or spill, and sometimes not even then.
You should probably charge them up normally first, then check again. Remember those are probably around 150amphours @ 20 hours rate. Daily self discharge is probably close to 2amps.
"My thought is to run all the lights and the furnace (sans propane), and let the batts go down from a full resting charge (approx 12.60 - 12.70V), go to .... where(?) in terms of Voltage. 12.15-12.25..?? to give them a 20-25% DOD"
I suspect the 12.6V-12.7V resting charge may be a surface charge, did you check that right after removing the charger or after they had been sitting awhile with out charging? Right after charging batteries will have what's called a surface charge which is higher than normal, it will go away after a short time and the batteries will reach a lower V resting charge. If your Specific gravity is low, and you have 12.6V showing as a charge, I suspect they may be sulphated and that you checked Voltage right after taking off the charger. I could be wrong perhaps.
I would run them on a desulphater or pulse charger for awhile.
I have a Vector charger and just bought a Shumacher a couple days ago. Both from Wal-mart. Both do desulphation, Vector I can select that mode myself, the Shumacher it only goes into that mode when it thinks it's needed. Not sure which I preffer yet.
If you got one or know someone with one, I would do that, or build your own. Somewhere here is info for building one.
I don't really know of any reason to run them low just to be doing it myself, but if you wanted to then I'd say around 11.5V should be ok. Don't know what % that is, but it's not far enough down to hurt anything.