I tended to “accumulate” things as I swung by the factory periodically, and one of the things I got was a new set of seat upholstery for my two (and only) seats. A year ago I bought some aftermarket foam-backed cloth covers that go over the original seat upholstery, so the need wasn't pressing, but I decided to put them on anyway and then reinstall the protective cloth covers. In the process, I learned how the seats go together, and what you'll be looking at if you decide to recover your original seats.
I pull the seats out, take the cloth covers off and throw them in the washing machine (that's the beauty of them – a quick wash and they look brand new) and set about figuring out how the seats come apart. The seat bottoms are easy – there are four clamps, two on each of the round bars the seat bottoms sit on. There's a U-shaped plastic channel sewn to the edge of the seat upholstery that fits along the front and sides of the formed sheet metal base, and another one along the back edge. careful prying with a blunt tool will release these. Before you pull the cover off, slide you hand underneath and release the Velcro strip that gives the seat its concave shape by connecting the cover to the foam. There's one across the middle of each cushion, bottom and back. If you mess up and tear the Velcro off the foam, you can always re-glue it.
The backs are a bit trickier because of the armrests. The inboard (aisle side) armrests are Mercedes stock, and they're secured by a bolt that you can access by unzipping the armrest cover and popping off the circular plastic disc over the pivot point. The outboard (door side) armrests are added by Roadtrek, and you have to know the trick. Look at the photo to see how they are secured. There is a pin on the armrest going into a hollow sleeve. It's welded to a plate Roadtrek pop rivets to the seat frame. There is a metal clip in a groove on this sleeve that catches on a groove on the pin. You have to sneak up between the upholstery and the frame from the bottom with a screwdriver and dislodge this metal clip before the armrest will come off. PLEASE convey this information to anyone working on your seats – it will save both of you a lot of grief.
Once you have the armrests off, pull the headrests out, undo the two interlocked U channel plastic things sewn to the back and front of the seat, release the Velcro halfway up the front, and pull the upholstery up to the top of the seats, exposing the headrest post sockets, You'll have to gently tap the bottom of these black plastic sockets to get them out. There's probably a Mercedes special tool to compress the tabs that hold them in, but you can't access them because of the design.
My seat upholstery fortunately came with no armrest cutouts, so I could leave the outboard armrests off by drilling out the pop rivets securing the plate to the seat frame. We never use them, and all they do is make it difficult to get the seat belt around where you can put it on. Every time I spin the front seat around, that seat belt gets lost. Now it's easy to put it on without opening the door and fishing around.
Unfortunately, my upholstery also came with bare fabric for the bottom covers, no plastic channel, so it was off to the upholstery shop to have them take the channels off the old seat bottoms and sew them onto the new ones. Regular sewing machines can't sew this plastic – you need a heavy-duty upholstery or sail-making sewing machine. If you're having someone sew you a new set, they can easily transfer these U-channel plastic strips from the old seats to the new ones. While I waited, I made do with a folding chair in the front. It was a little strange having no front seats for a few days.
I got to see how the fore-aft seat adjustment manages to make such small increments – a quarter inch at a time – by looking at how it's engineered. There are three pins, and they're positioned so that two of them drop at any given time. That makes three possible pin positions in the holes the pins drop through – here are photos of two of the positions. More German engineers with too much time on their hands, but it's a very ingenious solution.
My seat upholstery lasted almost five years of full time use and I expect more longevity out of this second set since we just got the protective cloth covers a year ago, too late to save the original upholstery from wear and tear and the occasional claw sharpening by you-know-who. The nice thing about the cloth covers is they have a foam backing, so claws can't go through them to the expensive upholstery underneath. They are available for $250-$300ish from Seat Covers Unlimited.