Consider an Arvika rv bike carrier. Mounts the bikes on the aframe above the propane tanks. Will fit up to 4 bikes have used one for 6 years works well.
Like the idea, don't care for the price. I know its Canadian dollars, but $300 for the base and $320 for the rack.
If you want to carry on back ...a couple of eyescrews into the "meat"of the corners and ratchet straps around the bikes to stop any movement and it works.
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It was a rainy few days at Algonquin Prov park...tarps kept us dry.
Wayne --Belle River (Windsor), Ontario
2013 Sunset Trail Reserve 25RB..(just sold it)
2015 White Chev Silverado 2500HD 4x4 (6.0 l gasser)
2003 Mountain Star 890SBRX Truck Camper