Watt Hour Capacity of 27 Deep Cycle battery

Watt hours is irrelevant. Amp hours is usually used to describe deep cycle battery capacity. It looks like the one you are referring to is a group 27 since that number stands out. if so, that battery is a little lacking. A group 27 battery has aprox 105 Amp Hrs of capacity. You can take the current consumption of all of your DC loads, multiply by the time it actually runs and figure how long you can run to get down to 50% charge. I'm not a fan of Interstate batteries, The marinas and repair shops around here all sell them and they replace a lot of them on a yearly basis. I personally prefer the no-maintainence batteries..JMHO, YMMV.