David,
Look at Walmart or a trailer specialty store. They do make adaptors. I suspect your missing pin will most likely be the reverse lights. It wouldn't take too long to wire up the proper 7 pin RV plug on there. I would guess less than an hour, then you would be done with it. You will probably end up playing with the wiring anyway, I have never some one elses wiring job match up with mine...
Aaron
