A single sign saying authorized vehicles only with no direction does not mean no parking anywhere.
The sign said authorized vehicles only beyond this point (the sign being the "point") and then you drove beyond that point in your unauthorized vehicle. I think that a single sign telling you not to do exactly what you did is sufficient.

...where there was a realtor sign and an open house sign and a dirt lot where others were parked.
Other people doing it will not hold up in court as a reason why you can do it also.

there were no no parking signs anywhere
You already stated there was a sign saying no unauthorized vehicles were permitted. That means not permitted, period. Including if they are parked. You weren't authorized to be there to park, drive, or anything else.