Best tip for using mirrors is REFERENCES. You don't try to use your depth perception, you compare one object in the mirror to another object in your mirror.
Tires to tape, camper corner to cone, etc.
The wildcard in something large is the "swing" - which can be the tail of the camper, the tail of the truck, or the bumper of the truck.
Keep those 3 areas inside the space you have, point your camper's reference point to the point you pick out in the mirror, and keep checking all three!
When you are not sure, GOAL- as mentioned above.
When you are blind, GOAL and make a note of how far you have to go and you can use something else to gauge that distance (even a tree next to you). Once you go that safe amount, GOAL again.
When I get close and need to line up or move precisely, I position my spotter with instructions like, "When the edge of the slide gets to right here, tell me to stop"
Really helps line up to clear slides/popouts, have awnings clear obstacles, etc.
You'll get it. Once you get past the uncertainty of an empty rear view mirror ;)