Originally Posted by dfinn
(why is there an inlet hose inside the van, can't figure out why you'd ever want to use this?)
Side note, but to answer your question - at least as it applied to my 1997 SMB: On my rig the only outside fill was the "city water" connection, the one you hook a pressure hose to when you want water pressure and water coming from an outside source at a campground. Most RV's then have a separate "gravity fill" for when you just want to fill the water tank (and then disconnect and just use your internal tank/pump). On my SMB (and I think most others), Sportsmobile rigged up the city water fill connection to do double duty, presumably to avoid another hole in the side of the van. So, to fill the water tank from a hose, you connect said hose to the city water fill point, then turn that grey valve inside by the water tank, which "turns" the city water fill into a gravity fill and you can then fill your tank with a hose.
BUT, sometimes when traveling, you want to fill the water tank but don't have access to a hose. Or maybe the hose supply is not "good" water and so you want to fill from jugs I've also had it in small National Forest campgrounds that there is a water hose type supply, but it's not meant for RV's to use and so there is no way to get close to it. Then jugs work great as intermediaries. However, you can't really "push" jug water into that city water fill very easily.
Hence SMB provided the alternate inside fill. Mine was simply a threaded connection with a cap, and I made a short hose/funnel arrangement that I could attach when I wanted to fill via jugs. It looks like yours has a "permanent" hose attached (which looks fairly scuzzy, so I would want to replace the hose section - clear hose exposed to daylight gets that way, so you may want to use opaque hose, and/or make it removable so you store it dry/dark).
Anyway, it may be something you don't have a use for, but that's the purpose. When I first got my SMB there was just the threaded connection and a cap, and I didn't have the parts to connect to it, and I really wished I had as I made a long trip just not finding a hose bibb. Would have been nice to just be able to pour 5-10 gallons of jug water into the tank.