They won't be airtight but have lids that keep things on the outside from getting inside. They need to be able to vent while cooking with lid down. You can get storage bags for grills but they seem like a bit of a bother to me.
Everyone who has a Weber seems to rave about it. Next one will be a Weber.
We have a Coleman road trip grill and it is cheaply made and has a lot of dumb design flaws. For example, when packed up for storage, the feet on the folding legs stick down about an inch or so from the bottom and the grill rocks back and forth.
I have to sit the grill on a foam block at each end when in the pass-through. Another one is that the lid won't close when you put on the full size grate. The first one we got wouldn't work out of the box and had to take it back to CW. The guy at the counter said they are junk and recommended swapping it with a Weber. In hindsight, should have listened to him.
(Whew, now I feel better with a little anti-Coleman rant!)
I modified our Coleman so that I can plug it directly into the LP gas line underneath with a quick connect fitting and 25' hose. Very convenient. I'd do the same with a Weber if I had one but don't know what's involved doing it to one.