If possible, it's always easier backing in from the (left), driver's side. But if you have to back in from the passenger side, have a good spotter.
So, if the spotter is truly on the blind spot, and truly blocked by trees, you have a couple options.
1. Use walkie talkies or cell phones and communicate, communicate, communicate! You, as the driver, absolutely do not get in a hurry or try to out-think your spotter. Your spotter can guide you without them being in your physical view (at the end of the trailer behind the bushes). But you MUST follow their directions.
My wife is my spotter and I've instructed her to tell me which way the back-end of the trailer needs to go, not which way I have to maneuver the truck. This simplifies tremendously. She says, "left, left, left!" I know which direction I have to maneuver the truck. We've successfully backed into completely dark spots and between trees in absolutely unknown new campgrounds many times. It's not easy, it requires a 100% dependency upon the spotter and it means .... GOING VERY, VERY SLOW!
2. The second option. First... you as the driver MUST still follow the directions of the spotter. And you, as the driver, maneuver the rig only a couple feet at a time. VERY, VERY, VERY slow. Meanwhile, your spotter is running behind the trailer, then back into your view in your mirrors. You back up a couple more feet, they check and come back. It means a lot of running back and forth for the spotter, but it does work. And yes ... you'll be sweating bullets when you finally get backed in.
3. Remember.... not only do you have to back in, but you also ... eventually have to pull out! Sometimes this is not so easy either. Your spotter is essential then too, especially if trees are close. Your trailer end can swing and hit something, long before you realize you just taken out the end of your camper. So your spotter is your best friend.
Together you can do it. Just get the commands down so you clearly understand each other.
(It's because of the many times we've arrived after dark at so many different campgrounds, we've made a commitment to arrive before it's completely dark. This makes for a shorter driving day ... especially between December and March when we like to do our best Southern USA camping. But's my wife has really become great with the instructions. We now use walkie talkies. They are are much louder than the cell phones and work in areas where cell phone reception is questionable.)