The RER is Paris' metro commuter railroad, with lines going through the city itself into many suburban areas. One line (the red one) has it's last stop on one end at DLP called Marne-La Vallee Chessy (Parc
Disneyland). Check out the
RER Map. The DLP stop is towards the right. Charles de Gaulle airport is on the end of the blue line also towards the right. If you wanted to take the RER from CDG to DLP, you would have to go into Paris first. The trip from the center of Paris to DLP is normally about 30 minutes, so if you have to go from CDG to Paris to DLP, I'd guess the trip would take at least one hour. From the center of Paris to DLP, the RER costs 6 euro each way, and is by far the cheapest way to get there, and is very safe and easy (the only easier way is a taxi, which would be MUCH more expensive).
Each RER stop is also a regular Metro stop. THe Metro is the subway within the city itself, and has many more stops within the city than the RER does. Here is a map of the
Metro and RER together. The lines with letters rather than numbers are RER lines, and have arrows at the ends to indicate that they go further than the confines of that map. This map is best for seeing which stops are both Metro and RER stops.
I can't help you too much about accomdations, sorry. We've only stayed at hotels within the city of Paris, once on-site at DLP, and only one night at an airport hotel at CDG. The airport hotel we stayed at is not meant for more than a one night stay, so I would not recommend it unless you are looking for an inexpensive place to spend the last night before going home on an early morning flight (the Hotel Ibis, for the record).
For one-day tickets, there are two options. One is a regular one park ticket, which as of a few months ago was 42 euro, I think. Then there is a park-hopper for 49 euro. For a one day visit, I recommend going to Disneyland Park only and getting the cheaper ticket, and holding off on the Studios. The Studios at DLP are fun, but on the small side, and it's easy to fill a full day at Disneyland Park.
Oh, I think you can get discounted tickets if you have someone who is an AP holder get one for you, but considering how few US residents have APs for DLP, you'd probably have to ask someone on this board for help there.