As mentioned above, the most important thing is do not be in a hurry.
Here are some of my tips:
-When you are lined up, you should NOT be able to see the rear jacks. The front jacks will be blocking their view.
I like to close one eye and make sure I am spot on.
The further away you are from the camper when doing the initial alignment the better. This will give you more room to make minor corrections as you make your approach.
-look for even spacing. Ie. 2 inches on each side
-Having the camper up higher typically makes it easier
(I put marks on my jack stands to ensure I have cranked them up high enough)
-tuck the wiring harness out of the way
(You don't want it to snag up while backing under the camper.)
-When you are backed approx. 2/3 of the way under (before you back under the wider rear portion) get out and check how things are going. This is when I hook up the wiring harness anyhow. Check how your alignment is going, whether you need to make a steering correction to the left of the right. (If you are out to lunch, pull forward and try again).
Again having the camper higher typically gives you a bit more grace because your truck box is typically narrower at the bottom with a slight curve to the bottom of the box,
Also having the camper higher makes it a bit more tippy. You might think this is a bad thing but it isn't, it gives you room to make corrections. You can slightly edge it over if needed or lower one side a crank or two and let gravity help center the camper.
-One other thing to look for is the the rear end of the camper is not too low. Just because the front end made it, doesn't mean the rear end will. Go to the back and duck down to ensure you have the vertical clearance on the rear end as well.
-Once you are underneath, you can once again use gravity to butt the camper up against the front of the truck box, by adjusting how far you let the front end down in relation to how far you let the rear end down. The further you let the front down in relation to rear will move the camper forward, toward the cab of the truck. You can also do the same thing to compensate for slight driver side/passenger side alignment.
-Your camper is going to settle. Don't worry about it being perfect. Tighten your turnbuckles, but don't over tighten them. You want a little bit of play to allow the camper to settle. Drive about 50 km and then check them again for proper tightness.
BTW, I've never had a spotter help me. I park my camper in extremely lumpy grass in an older back yard. I have to drive over and old sidewalk that is about 3 inches higher than the grass because the earth has settled over time. I've also loaded up the camper a few times in the winter on icy lumpy conditions. Where the snow is a few feet deep and I shovelled a path to the camper.