tortron, Because I noticed the same with my brakes when loaded I'm swapping pads to the more aggressive, and frequently recommended Koolstop Salmon pads. Another thing that may help are to clean the braking surface of the rim with a fine 3M Scotchbrite pad and a little acetone followed with a clean water rinse. You may need to deflate the tire to prevent it's contact with any acetone.

Be sure to let us know how the trip goes.

Brad