The access door is just that, gives you access to the inside of the truck bed. I use mine for a variety if things, depending on where I am going. I have used it for storing canned goods, shoe storage, empty cans, extra water hose, elec cables, and the list goes on.
As for the gap between the wing and the top of the bed, I put 4 4"x4"s (about 5-feet long) side to side in the truck bed. One in the front, one in the front and one in the rear of the wheel well and one in the rear. That gives me 3-1/2" more clearance. I have even used 4x4's on top of 2x6's for more height. It does not have to be treated lumber. I have tried every method I have ever heard of, but thus is the simplest, and I can use the 4x4's to support the TC when off the truck.
I think maybe the cement blocks you have under your TC is not a good idea. They are not designed to carry a load, and are subject to cracking and falling apart. If you really want to use them, put wood between each block and what it sits on. That will lessin the likelihood of their splitting.
Personally, I would never consider wood saw horses. I have steel saw horses I have fabricated. If you do a search on the forum, you can find info on using at plywood sheet "X" type camper supports. Easy to make and store.
Wayne