I took out the center section of removable floor to inspect the tanks and drainage. Is there any particular kind of sealing product I should use to put it back together? The old stuff seemed like plain silicone caulk. Thanks

I would suggest 3M 4200 If you want to remove again in the future.
3M 5200 if you never want to open it again.

Life Seal, 3M Marine Sealant or the like. I wouldn't use 4200 as it is still an adhesive.

you have enough surface area the even with 4200 you may never get it open again

I'd like to say I'll never open it again but I know I'll have to so that definitely rules out 5200 and I guess 4200 as well just to be safe.