I see I'm a bit late to this party but............
I think you are fighting a losing battle here, trying to operate high current devices from 12 V batteries.
Adding a second battery would give you 10 minutes of cooker operation; still not near enough.
Some things are just NOT practical in an RV. Converting high energy appliances to propane might be the only practical solution.
(Now to see what the other replies said!)
From your last post: Cooking your dinner on the way home ?? Really ?? Seems like you are going to a LOT of trouble just so you don't have to wait 20 minutes for it to cook at home.