RCI will deduct the gratuities from your on board account. However, you instruct them not to make the deducation and make them yourself.
You will have a form a few days before the end of the cruise which you can complete if you don't want them to deduct the gratuities.
Yes, it is a cashless system - all charges go on your SeaPass acct. All drinks purchased incur 15% gratuity.
No, there are 3 ways to pay the staff tips,  pay up front with the cost of the cruise  pay cash to the individuals or  get the tips added to your SeaPass acct - they are not added automatically. One evening you will get a form in your room asking how you want to pay them.
Mr John - RCI do NOT automatically put the grats on to the account - you have to fill in the form telling them that you want them to put the grats on. You have it the wrong way round.....
Thank you Aldrich and John, I thought that might be the case but having only cruised with Princess before, I thought it was worth checking. Thank you for the tip about opting out of the automatic deduction, We were well into our second week on the first cruise before we found out about opting out (Princess don't broadcast the fact).
In October the RCI policy on gratuities was to opt out. They also required all passengers in the cabin to sign the opting out form.
I know because it happened to me on the 6 October 2007 sailing.
Best you check when you board what the current policy is. I can only say what it was last October.