I assume it's a not a cabin RIB we're talking about here? There's been various threads in the past with people showing their sleeping/tent arrangements on open RIB's - I recall some very nice tent type setups, but typically on 8m+ RIB's.
I haven't yet tried sleeping onboard the RIB - if overnighting I stop in a harbour where there's some accommodation ashore, or take a tent ashore with me. As long as you have a suitable space to lie down, you could use anything from a basic bivvy bag to a fancier tent or bimini arrangement. I guess it depends on your tastes, and the weather!
Just remember if coastal hopping, especially in less protected waters, that weather may force you to run slower than usual, so don't be too optimistic with speed/distance initially, and have a backup/alternative plan should you not be able to make it to your planned destination.