I purchased a locking filler tube and cap for the gravity-feed on my camper's fresh water tank. This sort of locking cap seems to be common on some RVs, such as Eurovans, but less common on US-made RVs. The filler tube/cap came from Amazon (look up "RV Lockable Water Filler Cap"), but if you use my design, make sure it looks exactly like the one I have - I cannot say if my design fits other types of locking caps, although it might. While I like the idea of locking my water fill port, I didn't like 2 things about the cap that came from Amazon - first, there is no way to get spare keys - I went to 3 different full-scope locksmiths, and none of them had blanks for this type of "Chinese made lock". While it came with 2 keys, if you lose them, it will create problems down the road. The other was that it took (IMHO) too much force to turn the key in the lock - which is a recipe for future key breakage when opening or closing the lock. The other problem I had was that I'd gone through the expense of rekeying all of the locks on my camper to the same lock and key, and now I had one oddball that used it's own, proprietary, can't-be-found key. So my solution was to keep the filler port (tube), and replace the cap with one that uses the same lock and key as the rest of my camper.

To use my design, you need an existing filler port tube, and a cam lock. The lock I used is an RV LOCK (https://www.rvlock.com/products/rvlock-cam-lock), which I have on all the hatches and main door in my camper. You can substitute any brand of lock here, as long as it's the same length, and the square end of the shaft is the same size (if your lock has a different size square, you can modify my design in TinkerCAD or other CAD program, by adding or subtracting to get the right size square hole in the locking mechanism).

Since the outside of the cap is exposed to sunlight, I went with PLA+ for durability and UV resistance. I also used PLA+ for the locking mechanism for strength, but chose to use a contrasting color in case it breaks - I want to be able to see the pieces if it breaks, and white would not be as good for that. Also, for strength, I used 1.6mm outside walls and 80% infill on the locking mechanism, and 1.6mm walls, but 50% infill, on the cap itself. There is slight pressure on both parts created by the 1/8" neoprene gasket that forms a seal.

I included a hole for a tether in the cap, since it's always a pain to find a good place to put the cap down. I also included some finger slots to help pull the cap off - not sure if those were necessary, but it doesn't hurt, and some of the competing options on Amazon had finger slots, which seemed like a good idea. But as I mentioned earlier - if you use my design, it needs to match my original tube and cap (without the finger slots), or I can't say that my design will fit another configuration of tube. Actually, since I cannot control what Amazon sells, you just have to realize that what I designed fits my tube, and anything else is just gravy, including those who use my design for Eurovan and similar European RVs.

I also included a template for cutting the neoprene gasket - I traced it out on the neoprene, and then cut it out with scissors. This is simple, sticky-back 1/8" neoprene that I found on Amazon. If 1/8" is too little or too much, you might have to find another thickness, or double up on the gasket, to get a little compression to seal the filler port from dust.

Included photos show the parts before assembly, the assembled cap, the old cap along side the new cap, and the new cap installed. The STL files include the cap, the locking mechanism, and the gasket template. Ignore the "rough" cut of the gasket in the photos - I replaced it with a nicer gasket after the photos were taken.

Since I just made this, I cannot evaluate how durable this is - but I designed it beefy, so think it should be a permanent fix to the old cap.

Hope this is useful to someone...