Parking at the resorts is free and allowed for guests with business there, which would include just going there to check it out. But you can't leave your car there and then go elsewhere, whether it would be a theme park or other resorts. Parking at the resorts is guarded and often limited to 3 hours - it's not intended for leaving your car there.
If you have a car, it would be so much quicker anyway, just to drive your car to the next resort you want to visit - much quicker than Disney transportation, in which you would have to transfer at some point in most cases. There is NO resort to resort Disney transportation - it only goes resort to park or DTD.