Search is often your friend.
You might have found this post from long ago, in which I quoted some figures from Goodyear's recommended "match your Marathon tire PSI to your tire loading in pounds" table:
http://www.trailmanorowners.com/foru...09&postcount=5
Per my discussion there (and confirmed by your experience of trailer contents getting rattled and shaken hard), the tires at 65 PSI were running too "stiff", with no ability to "flex" in response to the road surface and loading changes during turns. When the tires don't flex
properly, a number of bad things can happen. The TM manuals from my year, and at least a fews years afterwards, always said to use the maximum PSI on the sidewall. For many of us, that's utterly terrible advice!
Here's the Trailmanor-Relevant lines of data in that post:
1760 lbs <> 35 PSI
1880 lbs <> 40 PSI
2020 lbs <> 45 PSI
2150 lbs <> 50 PSI (max load and PSI for class 'C' load rated)
2270 lbs <> 55 PSI
2380 lbs <> 60 PSI
2540 lbs <> 65 PSI (max load and PSI for class 'D' load rated)
As you see, you should have been reducing the pressure to about the same figures as Maxxis recommends for your M8008 tires: 45 lbs, maybe even a bit less. Goodyear, however, adds another "rule" in the other direction: If you're going to drive down the freeway at speeds in excess of 65 MPH, you should
add 10 PSI to reduce excess tire squirm at the front of the tire, where the rubber hits the road.
I have a 2619 with the class 'D' 15" Marathons. Although it's the smallest TM model, and we don't have an awning, we always load it up really heavy -- about 1900lbs per tire, after the WDH has loaded some of the weight "back to the TM". Goodyear's tuning table thus recommends pressure of about 42 PSI, but I like to have a bit of a safety factor on the high side: I use 48 PSI, and that works really well. 65 PSI is WAY out of spec. (And probably dangerous, as I described in one of my previous posts. If you're curious, follow the links form the one above.)