Hi Dave,
Whilst agreeing with you about dc to dc chargers, they are IMO a definite step forward from the VSR dual battery systems, I am unsure why the time limit of 4, 5 or 6 days (presumably without driving). This would be dependent upon how much solar. If the amount of solar input you had allowed you to go that long, just a little bit extra would allow you to go indefinitely, which IMHO is far preferable ....... If space & weight allow.

Cuppa