HI yankeeslover, you are catching this at a good time. Yes, they are all made like that, and I just got done replacing the entire floor in my slideout due to rot from water damage. Aside from the lack of sealant, the real issue is the lack of any kind of drip edge on that trim. When the slide is out when it's raining, water than runs down the side wall follows that trim right around and wicks into the edge of the floor. The quick and obvious fix is not to leave the slide open in the rain....I don't leave mine open as much as I used to.
That trim does come off....put the slide halfway in, then go inside the camper and get that last screw out, then fully extend the slide and remove the rest of the screws.
I would use a good silicone caulk and put a heavy bead on the edge of the floor then re-attache the trim when the caulk is still wet so it smushes together and creates a good seal. Pro-Flex is good for around windows and such but I don't know if it's what you want here. Eternabond will seal it for sure, and I initially tried to seal my slide that way, but the tape was interfering with slide operation and getting bunched up. It may work on yours and if so I would add some after you caulk it.
The other thing you want to do while that trim is off is add a drip edge. This is only a thin piece that sticks down from the trim so water running down the side drips off and can't run around to the bottom and wick into the floor.
I got a piece of vinyl J-bead from the drywall sectition of the home center and used shears to cut off a 3/4" wide piece. I stapled it to the edge of the floor and installed the trim over it. Because of the way the factory trim has that slight "J" profile to it, I re-installed the trim upside down. Here are a few pics:
The second pic is a real close-up, the drip edge only hangs down about 1/4" or so, and now the floor stays bone dry. I've been meaning to do a write up on the repair but haven't had the time. The bottom of the outer slide wall was rotten too, along with part of the main floor inside at the edge of the slide, I wound up with a few months of work in that winter weather....