Well, we used to just sanitize our tank once a year and use a Brita filter pitcher.
Then we installed one of those round marine clean out hatches. The first look into the tank would have made anyone cringe. Sure, there was some slime, but also plastic shavings and stuff left over from installation, tons of organic debris that basically looked like a layer of mud on the bottom. Since the openings to the pump or drain are not at the lowest point since they come out 1/2" above the bottom, one can never truly get out all the nasty gunk.
Now I can get my arm into the tank through the port and reach all 8 corners to truly clean it out. Besides, regular Brita and those inline filters "aren't at all intended to "purify" water or remove biological nasties such as giardia." Also, think of all those toxic industrial chemicals, herbicides, pesticides, and pharmaceuticals, etc. that are in your tap water. There is enough **** assaulting our bodies and our environment without consciously adding to it.
Since we are almost exclusively off-grid when we camp, we wanted to be sure we could purify our water no matter where it came from (rain, pond, river, etc). We have since upgraded to a simple gravity-fed purification system for drinking/cooking and even use it in the house. Look into Berkey or Royal Dalton. Plus you just periodically clean the filters, no replacements to buy like those wasteful plastic Brita things! These systems are used by relief organizations like UNICEF, the Peace Corps and the Red Cross. Bonus, when the SHTF you are covered!