An inverter will drain you battery
quite quickly, depending on what you are going to power with it. A heater would be the worst, a laptop, not so bad. If you do install an inverter, It would be a good idea to install an accessory battery
bank as well. On the Express van, the diesel version has a battery under the van, behind the driver seat. That is where I would install an extra battery that is isolated from your starting battery with something like this:
12V Dual Battery Isolator | Princess Auto
That dodad will connect the accessory battery to the alternator, when the van is running, and disconnect it when not running.
Lets talk power. If you get a really good accessory battery, it will have about 120 Ahrs of storage. That is, it can supply 120 Amps for one hour, or one amp for 120 hours. This is at 12 VDC. Power = Amps x Volts. so the battery could supply 1440 Watts, for one hour, or 12 Watts for 120 hours. A 1500 Watt blow dryer will run the battery dead, in less then an hour.
It should be noted that you should never drain your 120 Ahr battery more then 60 Ahrs. This will extend the life of your battery greatly.
The cost of batteries, isolator, and inverter, will be more then double what a small noisy generator
would cost. It may be advisable to look at a generator
on the bumper of your van, instead of an inverter.