I'm not familiar with the Ratweiler. It it from Swamp Rat? I'm guessing it's pretty hard steel, but how thick is the edge?
I can understand why you'd want to keep a backup stone right with the knife. And I do love diamond stones. If space is at a premium, then the diamond rod would work fine. However, don't get so caught up on a sharpener that you forget about all the ways to sharpen without a factory made stone. Anywhere you go, there should be plenty of abrasive materials laying around. Hard rocks or sandstone can be rubbed together to get a flat sharpening surface, smooth river rocks for polishing, broken glass for steeling & scraping, sand spread on your belt or a piece of wood for coarse stropping, the dirty thigh of your pants for fine stropping, etc. You may want to also consider carrying a section of abrasive belt that knifemakers use for grinding. It folds up to practically nothing, and works very well on convex edges.