This is a subject I'm pretty passionate about.
I agree with the previous poster that the PUR faucet system is probably the least expensive. The next best system, IMHO, would be the secondary small faucet added by the kitchen sink with an under the counter filter. This can also be plumbed into your ice-maker.
After 12 years of RVing with 4 different MHs we nearly full-time now, I have experienced lots of different water quality around the country, with northern Indiana being one of the worst (my daughter lives there.) So, my first attempt to solve this problem was to install a triple canister filtration system. This included two canister filters in the water Bay (10 & 2 micron) that filtered water going into the freshwater tank, plus the small faucet and filter system in the kitchen. The carbon filter did a good job addressing the varying tastes of the water at the kitchen sink. However, the dual cartridge system did little for the minerals and rust in the water. So toilets, showers and sinks everywhere needed constant cleaning.
Nine months ago, at the advice of a friend, I installed a whole house Reverse Osmosis system that fed into the freshwater tank. This was purchased from RV Water Filter Store: Standard Filter Canisters for Whole RV
and included a three canister filter system (10 & 2 micron plus a carbon filter) and a single RO membrane canister. I had to tap into the freshwater supply, the freshwater tank fill line and the sewer discharge line. The total install took me about three hours and cost about $200.
The results were amazing! We now get highly filtered and consistent tasting water everywhere. With the minerals and rust removed we get no spots or film formation on the toilets, the sinks, the showers, dishwasher or dishes. So everything stays much cleaner. I now wash the motorhome using the RO water and no longer have to dry it. A big time savings! I plan on replacing all of the filters yearly which will cost me about $100. But that's a small price to pay for awesome water!
There is one downside. If you regularly boondock for a long time, this system will not be very helpful. You have to be connected to a water supply and sewer system for the system to properly operate.
If anyone is interested I will be glad to post photographs of my system.