In the episode titled Brady's Braves the Brady's leave their campground at the Grand Canyon and the station wagon is pulling a pop-up trailer and the roof rack is filled with camping gear. As they drive alongside the canyon for one last look the trailer and gear disappear.

When they leave the lookout point and head out of the park the trailer and camping gear are back.

http://www.youtube.com/watch?v=lITmcnzAkL8