A 72 hour treatment of RV-Digest-It followed by a treatment with "RV Probe Cleaner" will clean the tank walls. If there are still solids trapped against the low spots along the wall you may still get the 1/3 light on. Once you get to the 1/3 light, if you can not make further progress, you may need to remove the lower two (ground and 1/3) probes and clean with a long thin bottle brush to break up the hard crud that is shorting those probes.
Lou, Laura, & Freya the wonder dog
2008 GMC Sierra 3000HD Crewcab SB Allison Duramax
2019 Flagstaff 8529FLS - Pullrite 3300
HAM CALLSIGN - KC3FFW