A 220w solar panel on a roof rack will probably cause more load on the van's engine from drag when driving than it saves by generating power, as counter-intuitive as that sounds, though it really depends on the speed you're driving at. Remember that 220w is only about a third of a horsepower, and you might need as much as maybe 30-40HP when cruising on the highway. This means that, if you're getting 20mpg, the solar panel might save you 0.5% in reduced electricity load, which equates to about 1/10 of a mile per gallon. If you lose half a mile per gallon due to extra drag (which is reasonable and realistic), the panel is costing you 5x as much as it's saving. The added drag might even be more than that.
However, if you use it when the van is parked to run things, and the alternative is finding an electrical grid to hook up to, it may be worth leaving there anyway, despite it costing you money while driving around. Is it worth your time and effort to stash the panel inside, or to make it flush with the roof? Up to you.
Your van is almost certain OBD II, which became standard around 1995 and is still used. It's very remotely possible that it's J1939 or J1708 instead, but I doubt it. A Scangauge II runs about $125, and an Ultragauge is around $70:
UltraGauge OBDII Scan tool & Information Center
You could also use an OBDuino, which is cheaper but not plug-and-play. You need to splice into an injector ground and into the vehicle speed sensor line for this, and then it will require you to calibrate it over several takes of gas, keeping a close long on fuel consumption.