I've been somewhat reluctant to put just any water in my fresh tank. Way back when, we were in the UP of MI (where I typically still go) the water there has lots of iron in it. It stained the tank tanish/brown. I'm not suggesting that it's an issue everywhere, but I filter all the water going into the tank. When low I buy about 5-6 of those 2.5 gallon filtered jugs at places like Kroger, IGA, etc. All 6 cost about $18 to $20. That typically can tide me over if I run low. To use these types of jugs though, I found a plastic transmission funnel that is flexible works great.
One way I also conserve my fresh tank when boondocking is to bring along one of those Coleman 5 gallon water jugs and those solar shower jugs with shower head and hose. I fill them up at the manual water pumps at the state forest campgrounds and use it for handwashing, brushing teeth, showering, etc. Doing these extended my fresh water supply by about 5 days, and I didn't have to buy those 2.5 gallon water jugs. If those $10 jugs get stained, so be it....my fresh tank still looks the way it did when it was first installed.