You can also get filters already "Custom Made" with hose fittings at most RV stores, and I think Sears, Surprisingly the prices are not all that outragous at the RV stores.... I've priced simular filters (Ice maker filters) at sears (Since I filter the ice water in my house) and the cost was almost exactly the same as a simular size and specification RV filter.
One thing to know... If you put a filter in the line it WILL affect water pressure within the MH. Normally this is not all that much of an issue but if you have a macerating toilet (I do) then it will affect the pre & Post flush fill levels,,, Also if you want to take a shower it's a good idea to have some water in the supply tank and hit the pump switch.
One assumes that If I spent a lot more money on the filters (Got larger ones) this would not be such an issue
Also filters come in different "Grades" from simple charcoal jobs (Good, for what they are designed to do) to ceramic (Better) to reverse osmosis, to combination jobs that can darn near(DO NOT TEST THIS THEORY) filter black water into drinkable water (DO NOT TRY THIS) (Remember I said DARN NEAR, I did not say they could)
Of course the more you want, the more you pay
Water, like filters, comes in various grades... Some a simple sediment and charcoal filter is more than enough
Other places a good bacteria killing filter is handy
Others have serious chemical issues that may well overwhelm a simple filter
Some have combination issues and need the BIG filters
Montozuma will get his revenge if you don't have a good enough filter