Hey Jon, Talon has some points, but his numbers are a lot bigger than mine. I actually helped a builder hook up a system like this in his truck so he could have AC power out on worksites where no power existed. Don't use your vans battery for this, get a separate one, in fact what works best is a pair of golf cart batteries. We put two T-105 golf cart batteries in his trucks tool box, you have to have two because they are 6 volts each. Then we grounded one side, and connected the positive side back to the truck battery through a heavy duty 50 amp 12 volt relay. When he drives, he simply turns a switch in the cab and the relay closes, allowing all the batteries to charge while the motor is running. Arriving at the worksite, he simply turns off the relay, the two batteries in the tool box separate from the truck battery, now he can use them at will and never worry about the motor starting back up. Then we simply attached a 1500 watt inverter to the batteries in the tool box and plugged a 6 outlet power strip into the inverter. Most hand tools work fine on that inverter. The fridge you have will probably run on it too, my 5 cubic foot freezer runs on the same inverter in our garage. He can run a 100 foot extension cord around his work site and use is drill and hand saw, or a work light at night. You can also use the 12 volt power from the batteries at the same time, like running a radio or van lights. If the battery runs down too far,just start the motor back up and turn the relay on for a while.
Talon is right that the 300 watt inverter you are looking at will barely run a laptop, forget tools and fridges, but a 1500 will work fairly well, it did for us. One word of caution, DO NOT plug battery powered tool battery chargers into that inverter, like a Dewalt Drill for example. It's okay if the charger has a wall pack type of cord that reduces the voltage right at the outlet, but the Dewalt types use a sensative circuitry at 240 VAC that the inverter will burn up. I've only wrecked one so far, learned my lesson on that. Better to just run an AC drill on a long cord plugged into the inverter.
For added endurance, my friend added a small solar panel to the roof of his truck, a 50 watt model I think. Wired it straight to the golf cart batteries through an inexpensive diode, no charger controller was needed because the golf cart batteries can take up to 4 amps continuous charge even when full without damage. Just another idea for your van...Good luck, Rudydoo