I picked up all the must-use-daily things and store them in the camper....mostly items that are easily overlooked and forgotten...like toothbrushes, toothpaste, contact solution and case, pillows, etc.
I'll second the TP, but point out the RV kind breaks down better as it is made for camping and won't clog drains...I have heard that certain brands are ok if you use lots of water, but dealing with clogs in the camper doesn't sound fun...I'd rather pay a bit extra (Walmart carries it for about $2.50 for 4 roles).
I'd advise you take your first trip to a place either close to home or close to a Walmart or other convenient store..load up what you think you need and get what was forgotten.