The key phrase to throw around is going to be "like-kind exchange." I can't find any definitive IN BMV information on how this works with two private parties, but the allowance that dealers get only applies to trades of the same type. So for instance, trading a motorcycle for another motorcycle, or a trailer for another trailer.
There is a special allowance for trading a non-motorized RV for a motorized RV (or vice-versa) which is definitely applicable in your case. To strictly follow this would mean you would have to pay tax on the difference in value between your trailer and his RV, and he would have to pay tax on your pickup.
It's crap but with it being out-of-state I can see things getting weird. Maybe pick a nice-and-low "hell of a deal" price and go with that? Not as good as free but saves you some money still. I've never been questioned on a purchase price either, as long as they can say they did their job the employee won't really care, and if you are giving them a grand in tax they are more likely to ignore it than come hunt you down to ask questions.