I'm gonna be a **** here and point out that we all have legal responsibilities to keep our vehicles safe for us and the rest of the non-6x6'rs out there on the road.
That said, I point out that increasing the capacity of the braking system will run the compressor longer, building greater heat per cycle, and shortening its life (as was previously noted). If you run out of air because your compressor died, you have a HARD time stopping. Yes, more volume equals more stops before you run out of service air, but especially for those of us with steep grades (Cajon Pass, Beaumont Pass and Lamb's Canyon, the Grapevine, and etc.) running out of air will be bad for all in the vicinity.
Using air tanks not DOT-approved is asking for trouble. If you get into any kind of collision and they find non-DOT equipment (bet your ass there will be an inspection), the victims' insurance companies will own you and you may do jail time (and YOUR insurance company can probably wash its hands of you for non-DOT modifications). SCUBA tanks are not for use as vehicle equipment. You can get decent DOT tanks at any truck salvage yard.
If you ARE going to add a tank(s), they should be connected via correct and standard tubing and fittings. Don't use galvanized water pipe.
The best idea was to pre-charge the system with "shore" air; at least the compressor won't have to work outside of its design to get you going in the morning.
Let's all be careful. I don't need "Bubba" screwing me out of my MV fun; neither does anyone else.
Sorry, not meaning to rant, just one of those uptight "personal responsibility" guys!
-Ken J