Couldn’t disagree more. Still got the same setup a few years in. As mentioned the sag is minimal (not non-existent), I place a small removable 2×2 near the tailgate so I can hop up top. You’re not building a home up to code, you’re building a practical truck camping solution, and in my extensive experience it is more than fine. You can do thicker plywood, but realize that it will be much heavier and more cumbersome to move.
Great shop just what you need. You mentioned your lighting was a little thin on the ends of your shop. The best way to lay out rows of lights in an open space like yours is to figure out how many rows you want. Now in your shop six rows would give you a good coverage, so you would divide the length, 60′, by the number of rows, this gives you the space between rows, then divide this in half, this gives you the space from the wall to the first row. For a 60′ room with six rows this would be 10′ between rows and 5′ off the end wall. This will give you consistent light from wall to wall.
The refrigeration setup in your van is an area where you really do get what you pay for, whether you’re paying in money or in time. The best options are the most expensive. The cheapest options are a pain and/or don’t work very well. And if you try to save money with a DIY refrigerator setup, you could end up spending a lot of time on installation.