What's the current draw on the 6V with the converter operating? Unless you're running some speakers with an obscene wattage rating, you'll probably be OK. Another approach to power the system might be one of the small gel cell 12V batteries and a trickle charger, and/or a solar panel that can be used for a recharge of the dedicated battery when the truck is parked. That would allow you to run the sound system for several hours at a time, completely independent of the 6V charging system, and put no stress on it at all.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!