How to make walls thicker using meshmixer or meshlab
I figured out how to do it by using the make solid tool.
Edit: I selected the whole model using control and selected make solid. Set the mode from fast to accurate. I set the solid accuracy and mesh density to anywhere from 300 to 500. Then I slowly increase offset distance until the holes are sealed. I leave the minimum thickness at 0 because it doesn't appear to help much. When I'm satisfied, I click accept. I usually use the reduce feature to make the file smaller, but it isn't required. Sorry about leaving an unhelpful answer. Hope this helps people.
This doesn't help anybody who might have the same question in the future. Self-answering questions is encouraged if you figure out the answer yourself, but please explain your solution in more detail so others can replicate what you've done.
Please consider elaborating so that others with similar issues can learn as well.
The answer is low quality, but I don't think deleting it would help anybody, as the solution may really be as easy as to click on "make solid" somewhere on the interface... deleting this answer would remove the only valuable clue on how to solve the problem.