First of all, welcome to SDN! In the future, try to do a google search or search the forums before posting a new thread, as all of these answers are easily found and/or are asked often.
WIll I have to decline due to secondary applications?
No. Will you have internet access at all during the trip?
When are they usually sent out?
A few schools send them out when you submit your AMCAS (primary application), however most schools will send the secondary when you are "complete" - aka you have your AMCAS verified and your MCAT and LORs are submitted. The earlier you submit your AMCAS, the faster you get verified, and the sooner you get secondaries.
How long do you have to return them?
Some schools have deadlines (only one I know of for sure is WVU, but I know there are others). Usually a good rule of thumb to follow is 2-3 weeks after receiving it.
Are they online or sent in the mail?
They're usually sent via email or you are emailed with some kind of login information for the application website. There are a few schools that require you to print out the application and mail it in. Some schools require you to send in a 2x2 photograph. Every school will be slightly different, but I'd say the majority of them are online.