Drinking water for us always came from purified water bought in 5 gallon jugs, called "garrafons". Buy a couple of 'bombas" (simple inexpensive plastic hand pumps} once you get to a Mexican hardware store. The jugs have necks made in a couple of sizes, so select a bomba to fit snugly over the garrafon you have. If you wish to add purified water directly to your main tank and are faced with pouring into a vertically situated filler, select a 2 litro (approx.) plastic Coke bottle, fit the neck to match the filler hole and then cut an oblong-ish circle in the side of the bottle. Unless yours is a complicated system, this apparatus will direct the water in nicely, and spillage can be kept to only a small amount, or nothing, with practice. As well, water truck workers will sometimes add their water to your tank for you.
In many seasons of camping in Mexico we never had bad water from these containers. Filtering presupposes you will have a means of pressurizing the water to push it through to the tank, in other words, from the water supply at an RV Park, etc. Buying the water available in garrafons means you will have more latitude in where you camp, always with an eye on keeping foreign material from accumulating in your fresh water tank.