Hollow wing for rc plane
I am sorry to inform you, that the answer to "How do I fix the thickness" is "Remodel them" - especially in this case as the whole design is... awkward.

But you don't necessarily need to resign them from scratch, if you can fix it... But beware, fixing does only work sometimes...

First of all, Blender is NOT a good modeling software for designing parts that shall be printed, Blender is a 3D Artist program, not a CAD program. It can serve its course, but it can and will ruin your day. I suggest grabbing Autodesk Fusion 360, since it is mighty and free for small makers.

Step 1: Transfer into CAD software

For our first step, we want to take the surface of the Wing and export it as an STL. To do this, remove all interior vertices. ALL. Save as a work-project. Look for "BAD" areas - try to have as little vertices as possible. If several are in the same flat area feel free to remove some. The simpler, the better - compare these two pictures - left the bad side, right the good.



Export via File &gt; Export &gt; .stl.

Open Fusion 360 and import via the process outlined here: Insert &gt; Mesh.

Now we need to turn our Mesh into a BRep like described here: 


Deactivate the pickup of model history by right-clicking the project in the left, then "Don't capture design history"
Modify &gt; Mesh &gt; Mesh tp BRep
choose your object and OK
reactivate the model history by right-clicking the project in the left, then "Don't capture design history"


Step 2: assigning Thickness

We got a surface now... or rather several that are stitched together. We want to give them thickness...


create &gt; thicken
click on one area, choose the thickness as a negative value. For example -1 mm
click on the body's lightbulb to make it visible again
rinse and repeat for each area not yet thickened


Hint: rightclick opens a context menu that offers repeat ..., where ... is the last used operation, in this case: thicken. This considerably can speed it up.



Step 3: Combining thickened parts

Now, we have several thickened parts, all of them intersecting or touching. like, what usually looks like this...



actually is these different parts (which I colored for showing only - it is totally unnecessary!)


It's easy to see these all intersect. And luckily, intersecting parts can be easily merged! 


Modify &gt; Combine
click one, then another. OK.
rinse and repeat as much as you can - some pieces will throw an "inconsistent edge-face-relationship" error. If these crop up, you need to start over, fixing the Mesh.




To state it clear after wasting 2 hours on this:

Your files needs to be done again from scratch.

In a proper CAD modeling software. Because what you have there is not fixable easily.