I have the BMV-712, and I imagine the software is similar to the Smart Shunt. 60% from 100% is a big change. SOME drift over time is normal (temp changes, slight inaccuracies in Peukert calculations, etc). Even a small parasitic load like the Propane Detector (often hard wired in with no fuses, direct to the battery for safety reasons) can create a mismatch.
I do like you did: after a trip I plug in to shore power and have the camper in the sun... After a few days I call it good and SYNCHRONIZE the BMV.