According to Michelin's website
http://www.michelintruck.com/reference-materials/manuals-bulletins-and-warranties/load-and-inflation-tables/#/65# in the front tires is enough pressure to support 4,670#, slightly above your GAWR, so at 75# you're over inflated.
55# in the rear is only adequate for 7,500#, so you're most likely under inflated.
You should have your MH weighed, all set for a trip and adjust your tire pressures accordingly.
Your MH cams with load range D tires, which was the norm back then. They now come with load range E tires.