Here is a post I made here a couple of years ago:

Hello,
I've raised living coral for about 19 years, internationally recognized (and I brew beer ) so I'm a bit of an expert (ex= has or has been. spert= a big drip. Expert=a has been drip) regarding water purification. I haven't seen any specifications in regards to RV filters so I can't comment on them specifically but I can recommend some rather inexpensive things to look for. I can also give you a general working knowledge of how common household type water filtration systems work.

Generally speaking the first filter in a system is the particulate filter. This is the least expensive filter but does the most to increase the lifespan of any/all filters downstream. They can be found on Amazon for .99$-13$. A cheaper filter is the 5 micron (typically found in inexpensive systems). Runs .99-$1.37 on Amazon and the 1/2 micron runs $4-$13. Neither of these filters will reduce mineral content enough to prevent all sediment from forming in ice cubes but rust from the pipes, and any larger particles will be caught. I have noticed that on RV's there are typically only 2 filters with the second one being a carbon filter. A good carbon block filter in the 1/2 micron range runs $10-$15. This filter combination will give you safe pretty good tasting water. Obviously a 5 micron filter is going to allow 10 times the size of particles than a .5 micron filter.

I used to own a Salt water aquarium store (12 years) and sold RO/DI systems to provide pure water for aquariums. RO= reverse osmosis and DI= de-ionizing. I was often asked how long will the filters last? Well that depends.....mostly on incoming TDS (total dissolved solids). These run from 15 ppm - 450ppm depending on the water source. A typical aquifer/well type water supply will run 100-150ppm. Systems relying mostly on rain water will run as low as 15 ppm and are very good for building your own water for different styles of beer . In the worst case (high incoming TDS) filters need to be replaced after 750-1,000 gallons are run through them. A low TDS water supply can produce more than 5,000 gallons. A good DOW brand RO filter runs $30-$40. A digital TDS meter good enough for the home user (+-5ppm) runs about $30-$45. It's great for testing fresh water supplies to know what your dealing with.

Well, are you bored to tears? If not continue reading, you will be

If you really want clear ice you will need an RO membrane which consists of a tightly wound membrane with holes about the size of a water molecule. A good one will typically have a 98%+ rejection rate which means water is pretty darn clean as a result. Waste water is 3-4 gallons for every clean gallon which results in quite a bit water to deal with (good for watering the yard/garden or feed down the waste pipe). A typical home RO membrane produces 50-75 gallons/hr. Too slow to directly feed a faucet. A 6 gallon bladder tank is normally filled and supplies a water system for quick delivery (of up to six gallons). The typical production of a three filter system let's say 1 micron particulate, then 5 micron carbon block, then RO membrane with a starting/input TDS of 125ppm will yield about 3ppm output. This will give you quite clear ice and tastes great!

If you are growing corals, a beer brewing nut, or do lab work in your spare time read on....
There is one more filter. The DI filter which is made up of a mixed bed of ions and cations beads that attract the elements electrically. Water passing through this media results in 0 TDS which BTW is not electrically conductive (how weird is that?) and does not occur in nature. It is equivalent to distilled water. For the ultimate in clean water a 4 stage system is required. Note: DI water is not healthy to drink in any quantity and doesn't taste good (You only thought cleaner was better).

Most people like drinking water in the 5-50ppm range with some combination of magnesium chloride, magnesium sulfate, sodium carbonate, calcium chloride, or other salts for best taste.

Hope you're still awake

Cheers,
Kevin
PS: Very good information Chemist308.