they sell stuff in strange places called shops, you don't need to take much in the way of stuff. As other have said, tent, sleeping stuff and a few bits and pieces. AA card, money and a sense of humour.... will see you through the rest
If you fancy some dry bags l've started using LOMO ones strap straight on the panniers and cheap with really good quality
As everyone says you tend to find system that's suits the way you like to travel