Ark Power Pack
We have one with a deep cycle battery. Whilst I organised my hubby to buy it, I don't know enough to compare it to a dual battery system. I can tell you we have had it supply power to a fridge/freezer, our LED light system for the annex (seriously, it's like a disco/party light!) and charge phones & iPads (x3) over the course of four days and it hasn't even looked like it worked hard.
That said, we have two solar panels hooked up during the day and the fridge on charge during the drive.