Wet OSB will work for years, it will take a while for it to rot away.
Leave that project for years from now.
Your rear tie-down system is fine. Not as good as the frame mounted one, but it will suffice.
Depending on the weight of the camper maybe you could get the super cheap tiedown mounts that slip into the rail holes on the truck bed rail. Few will recommend these, and it is the most likely to damage your truck bed, but is cheap at about $60 for some used ones or around hundred n change for new.
Torklift mounts are very good, but will run you $300-400 bucks for just the front pair, and requires some tool ability to install depending on the particular truck in question.
The tie down chains should be at an angle and oppose each other from front to rear such that one pulls the camper forward and the other pulls it backwards. This keeps it centered front/back .
The tiedown chains just get tightened hand tight to where the chains are snug. Pluck them like a guitar string and they will make a "boing" sound just barely. Not tight, too tight and bumps on the road can damage the truck or camper. Don't tighten one side too tight then have no room to properly tighten the other side equally.
*Though, the way your rear system is set up you may want to tighten it more as i suspect that the long square tube from the trailer hitch will have a lot of spring in it and be very bendy under load, so that bending will protect the camper tie-down mounting point, so possibly tighten the rear more than the front. Test it how much it will bend and use judgement.
If that guy is going to help in person then you really do't need more attention here…