We have always used a system that does increase our water supply. We have never used it to increase the fresh water tank capacity but I guess you can say it extends the supply.
Since we live on our ranch we have our own, private, well. Our water is clean and pure and has no additives, such as cholorine, etc., that is in city water. We like our coffee without the Chlorine, etc., taste that is always in water in restraurants, RV Parks, city homes, etc. So we always bring a couple of gallons, or more, depending on the trip length, of our own pure water from our ranch well when we travel in the RV. We also fill our Fresh water tank with pure well water but when we camp in an Rv Park we hook to the park's water system. We use the park water system for washing dishes, showers, toilet, etc., and use our pure well water from our fresh water tank and/or the gallon can(s) for cooking, coffee, and drinking.
On extended trips when our fresh water tank gets low, and we refill it in a RV Park, we use the water in the gallon containers for coffee, cooking etc. and the water in the fresh water tank for all other. In these situations when we get back hom I always completely drain the fresh water tank and rinse it out with our well water before our next trip.