The customs folk are generally looking for proof that you aren't going to stay here illegally. When I used to come here from England to tour(I now live in Colorado
they would ask...
where are you going?
how can you afford to take 3 months off work?
did you buy anything in Canada?
any Canadian marijuana? apparently it's better than American weed
do you have any weapoins or bear spray?
are you carrying any citrus fruit or bananas?
I've crossed the border into the U.S. 3 times on tours and these it seems are the standard questions they ask everyone. You will need to have with you...
your maps to show where you are going as you have no address of anyone you are staying with or where you can be contacted
your exiting plane ticket
your passport and visa/waiver
probably a copy of your last bank statement to show you have enough money to survive for 3 months without working here
you may not be asked for this stuff as this is what you usually need to get a waiver but if they are having a bad day and ask you and you don't have it you may not get in
also, if you meet someone and are riding with them it would be a good idea to cross seperately as you never know what they are carrying or what kind of record they have...I met a Canadian who crossed 5 minutes in front of me in his car and he got his car emptied...everything was out on the floor...he didn't get in, I did
good luck on the GDMBR and have fun, it's amazing.