Welcome to the forum.
First, try what Coldfusion suggested. If those haven't stopped enough of the shaking for you, the next steps would be to try adding jacks near the center of the trailer frame and/or 'Strong Arm' type (there are a couple of different brand names) brackets and rods that tie the jacks together diagonally.
Sherm & Terry w/rescues Steve (std Poodle) & Eydie (min Schnauzer)
2015 Jay Flight 27RLS (Camped: 102 nights 2015, 90 nights 2016)
2006 Ford F350 6.0 PSD, Lariat, 4WD, CC, LB, SRW, auto., Camper pkg.