Your Multi-Use battery is basically a starting basttery with lip service to Deep Cycle mode.. A true DEEP CYCLE battery is better for house use. but may not have the OOMPH to star the van. a dual battery systems addresses these issues.
Some numbers. The "Group" numbers are MARINE/deep cycle dual purpose so you should not use more than 20-25% of the capacity.
Group 24, 75 Amp Hours
Group 27, 95-100
Group 29, 100-105
Group 31, about 130
8D or 4D About 220
Now GC-2 (6 Volt golf car, you put two in series to make 12 volts) 220 amp hours and you can use half of it.
Your 700 watt cooker draws about 70 amps of battery power So 15 minutes is.. actually rather long for a single 12 volt MARINE/deep cycle.
NOTE that the faster you draw them down, the faster squared they run down (not an exact ratio) but the capacities I gave are the C/20 rate (that is a 100 amphour battery can provide 5 amps for 20 hours before it hits zero).
If you draw current faster.. Well logically at 10 amps that 100 amp hour battery should last 10 hours. but .. in reality, due to an effect I can never rememeber the name of |(Purket?)_ it may only last 5 or six.