When I die I accidentally went back to Camp instead of Reviving to the map and lost my backpack expanding..

Maybe that because you clicked random revive..

When you visit the postman you can see at the bottom there is a 'set revival point' button

You should tap on that button so that when you die..

You just tap revive and you will revive near the postman besides the helicopter..

And when you revive you will not lose all your resources..

Only a tiny bit percentage of them