A quick search here will reveal several excellent threads that discuss this at length. Many use the Geo-Method and some use a combination.
I use a modified geo-method (Liquid Calgon and laundry soap) every dump and use a 1 Quart 72 hour RV-Digest-It treatment every season followed by a long Turbo-Flush every end of season.
My tanks are like they were when delivered and, with the Horst probe upgrade (also here in the FAQ), the indicating system works perfectly every time.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW