Hey moose, sorry about the pup, hope all goes well.
This may or may not have been suggested before, but Ill through it out there anyway.
I noticed you are running a 3B. If you don't have AC, you should be able to build a bracket for a large 12V alternator. (I just finished mine a week ago, ill post pics if your interested.)
I grabbed a new 130 amp Brushless Denso for $80Can off
ebay, and spent maybe $200 in material for the bracket ($150 of which went to Toyota for their crappy pulley)
You would then have an isolated 12V system, attach a capacitor and there you go. you could power a large system withought too much trouble. Granted the engine would have to be running, however this way you save the hassle of lost space with installing a third battery (this would be my primary concern,) not to mention without a new alt, how would you keep a third battery charged?
This way you also have 12V without having to tinker around with the 24v system and possibly draining your batteries after listing to too much satellite radio....
PS you could also wire in a SMALL 12VGel cell to your deck, attach it with a small relay, so when the engine is off the system would power down, but you would keep your presets in the memory.
-walk