I locked my valuables in the top box.
Otherwise when in France/Germany/Switzerland etc I left everything else on the bike in soft luggage unlocked, including my gear. No-one nicked anything - and tbh I would wonder why anyone would want to nick my clothes - for the whole two weeks. Not a chance Id do the same in the UK though.
But for your valuables - passport/documents etc probably a rucksack to carry with you? most tankbags have a rucksack zip on option so you get the advantage of both. Just a consideration.