Search RV-Digest-It on the forum or Google it for how to clean your tank without replacement.
As to the valve, they are easy to fix once you gain access to the valve if all that happened was the pin that connects the handle to the valve shutter broke or fell out.
Total replacement with a full tank is not to the faint of heart and most shops will skin you to do it.
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