I had the same problem.
I had an ENORMOUS amount of water in the tent from when it was parked in long rain storms.
Root cause was actually the rivets all over the roof.
(We had a very rainy winter here in northern California. 80-100" in some places.
I put my mattress pad in the garage to dry, as well as the sleeping bags.
I threw away a pillow and mattress cover due to mildew.
You will know its the rivets, and not the side seals because: water is all over the roof liner, the roof and base are lapped, so water cant get in the tent, even if there were no seals there at all.
I ended up getting a tube of clear silicone, and applying a patch to every rivet and screw hole on top, and the angled sides of the roof. There are about 40 of them.
ONLY do this when the tent is dry, and it is not about to rain. If the silicone gets wet before it cures, it will dissolve.
I did nothing to change the roof to base seals, and my leak problem is now gone. (Lots of rain before and after the fix).
There are 2 rectangular support channels down the center of the bottom of the tent. I had 2 more problems with these.
1, they fill with water. when you drive, the water sloshes out. I drilled small drain holes at both ends of the base of the channels.
2, They do not support the weight of the tent with people in it adequately. My tent was mounted at the ends. (not in the center, and not using the brackets it comes with). Those rails, and the whole floor of the tent had yielded and was sagging 1/2" (With no one in it). I had to raise the tent off my rack, and weld another cross bar in to support it. I might be pretty fat at 230lbs, but I would expect more strength than that. (Or mayby some instructions indicating how it needs to be supported).