I’ve done plenty of long distance multi-day stuff with a rucsac, specifically a WingNut MPS which I seem to remember has a capacity of approx 40/50 litres.
I still strapped my sleeping bag under the bars, and used a saddle bag, but most of my kit (including food for 3-4 days) was in the RucSac.
If you don’t have a wingnut you might have problems with the rucsac hitting your helmet, plus the ergonomics of the WingNut (i.e. very low) seem to improve comfort.
Would agree with TJ on this one, from a comfort point of view it might make sense to have weight on the bike, but from a handling point of view on tricky rocky stuff I much preferred taking weight off the bike, even if it meant placing it higher on me. Think somebody explained to me that your body moves in a pretty smooth line, no matter how tricky the trail, but the bike moves loads, which is why it makes sense to keep the bike light.