Congratulations and welcome to the site! Good choice on the camper, it will give you lots of flexibility for storage and bikes.
The ADCO covers are good to use, if you just want to cover the camper and not the front storage rack, you will need the 12-14 model. I would guess that the next size up would cover everything. When you cover the camper just makes sure you pad all of the sharp corners, you can use foam noodles or pipe insulation. I would also recommend making sure that no straps are laying on the ground and all gaps / holes are sealed underneath the camper, Critter Control.
Coupler locks are a good idea, but make sure you have a couple lines of defense against theft, no matter what you do, if someone wants it bad enough they will find a way to take it. I would get a good thick cable and lock and if you can, secure the camper to a permanent structure or run the cable through the wheels and to the frame. This will at least make it hard for someone and deter the lazy thieves.
This site is full of campers willing to offer up their knowledge and experience with the A-frame campers.
2012 A122 (SOLD)
2016 A122 SXR (New)
2016 Toyota Tacoma DCLB