Sorry for the delayed update.
-I installed a 1.25" pipe plug in place of the overflow.
-Tank was completely drained, same procedure as before.
-Using the gravity fill outside the rig, was able to fill almost 1/2" higher (74.5 gallons)in level before the sound changed (water level rose above vent fitting) -I continued to fill for a bit more, but stopped when water started to mildly spit out. I honestly didn't feel comfortable taking a chance to pop a fitting off and get everything wet under the bed.
Ended up with the overall level 1" higher than before the plug was installed (77 gallons) It looked like there was another inch of air in the tank.
That additional inch should give another 5 gallons if filled. It looks like the tank might hold 81 or 82 gallons with no vapor space in the tank.
Now for the good news!
I drove the rig to run an errand and there was no overflow sloshing! This is the first time I haven't left a snail trail after topping off the fresh water tank. Pretty stoked about that.
There isn't all that much to be gained in capacity by plugging the overflow, maybe 4 or 5 gallons, but the lack of slosh loss should save at least that amount. I think its probably a 8 to 10 gallon difference from fill-up to camp. That's worth the cost of a dollar plug and some PTFE tape and 15 minutes of effort.
It's still a bit of a hard pill to swallow that there's no way to get much more than 80 gallons into a "100 gallon" tank.
I'm not sure about the accuracy of the meter I used, so I would encourage others to do their own testing. Maybe a factory test on an uninstalled tank?
The numbers from testing align with the strapping chart calculations (~4.9 gallons per inch).
Please ask questions and comment with your own experiences.
Sean