I see lots of good advice above!
First and foremost is to follow manufacturers instructions. Follow coach manufacturers instructions. Follow electronics manufactures instructions. Follow chassis manufacture instructions.
Disconnect shore power, shut down engine, switch off solar and wind controllers. Remove all sources of 12 volt charging.
The general safety rule is to remove the negative cable from the batteries first. This prevents accidental sparking short circuits, blown fuses, tripped circuit breakers, and damaged wires and connectors from a loose hot positive cable. Removing positive first requires careful, disciplined, skilled action.
As added security, I always put a plastic bag over the loose ends to prevent surprises. Wrapping in a shop rag also works.
House battery bank:
However, some electronics including many Magnum inverters require removing the positive 12 volt power from the Magnum first. Removing the large fuse in the Magnum 12 volt cable does this just fine. Then remove the negative cable from the battery bank.
Engine battery bank:
Follow chassis manufacturers instructions. There are lots of different electronic components that have different requirements. Sometimes you need an additional battery connected to support "always on" electronics.
Sometimes the house and chassis battery banks have been "hot wired" together. Verify banks are separate or remove both negatives before proceeding.
Modern systems are highly complicated. Accept some risk of failures during this procedure.
I wish you good luck and happy trails ahead!