Tom...We have an 08' 3600RL, essentially the same unit as yours. We originally had the longer Trailair Pinbox and had no problem hooking up. The unit went back to the factory for a problem and Heartland changed our pinbox out for the shorter model. This was done to help eliminate the extra length which was partially involved in a "flex" problem...too much extension is not good. When we went to pick it up we had the same situation you are now experiencing.
The solution was simple, we opted to use a "V" gate...no more problems. We have absolutely no turning, backing issues and hooking up is actually MUCH easier. We've used this combination for several years now. As long as your rock guard does not stick out past the edge of your bumper on either side, you too should be good to go!!