What if I am only running the inveter when the van is running?
That won't really change the design of the battery. You'll still draw stored electricity from those thin lead plates in the cranking battery, and the alternator will quickly recharge the battery in the process. So you won't run the starting battery down while the van is running, but the battery will still need to be replaced long before it should, because the rapid discharge and recharge causes premature sulfation buildup, which is the death knoll of a battery. Everything will seem fine, then one day you'll go to start the van and the battery will be dead as a door nail. And it won't hold a charge. You'll jump start it, and it'll run fine all day long, but as soon as you turn it off you'll find it won't start the van because it's dead as a door nail again, despite having been being charged all day by the alternator.
There are basically three types of batteries that work in various applications for us out here. One is the cranking battery, which is designed for lots of amps in a very short period of time, and will recharge rather quickly.
Another is the deep cycle marine, which is designed for the low amp draw of a trolling motor or boat marker lights (about a 5 amp sustained draw over a period of a few hours), but can double as cranking batteries (marine deep cycle and "truck" batteries are virtually identical). These work well for low amp draws and discharges which are not very deep (no deeper than 50%, for sure, or until the battery voltage is about 12.2 volts). They are not at all suited for microwaves, refrigerators, heaters, air conditioners, rice cookers or any other inductive heat appliance. But they will work fine for a laptop, couple of lights, certainly an Espar heater, and maybe a TV or something.
The other kind is true deep cycle batteries, AGM or wet, generally much more expensive than cranking or marine (truck) batteries, and will handle higher amp draws for longer periods of time. Not designed for cranking, but can be used in a pinch, especially in larger battery banks.
Pick the battery for what you want to do with it, and don't try to make a battery do something it's not designed to do. The old saying, "Batteries don't die, their owners kill them," is very, very true, because people are constantly trying to use a battery for something other than what it was designed for, or they fail to properly monitor it and end up routinely discharging it too deeply, and then not fully recharging it. Running a battery down to where inverters scream and lights go dim, and then running the engine for 15 minutes to recharge it is First Degree Batteryslaughter.
And like Dennis said, there is no cheap, easy way to do this. If you cheap out, you'll have nothing but problems. You want to use the right cables, the right lugs, the right fuses, and the right isolator (or separator).
