Found it! The screw that holds the ground terminals to the frame had broken off inside the frame. It was still threaded in far enough to be somewhat secure but not good enough for a good ground connection. Thanks for all the help!
2010 Jayco Recon 40D
2008 Ford F350 Diesel Dually
2004 Yamaha Grizzly 660
2006 Bombardier Outlander Max 800