Timmac can't directly answer with the information provided. Specifically we would need to know the tire manufacture and tire size. Most, but not all tire manufacture have a table that will provide a load to pressure table on their web site. For example
HERE is a Goodyear chart, or
HERE is what Michelin published for example.
I recall reading how you beefed of the rear of you rig for the added weight of the trailer but I sure hope you have a Dana 145 rear axle and not a Dana 80. The builder's placard listed tire pressure is based on axle rating and the tire rating as originally installed. Considering your rig is build on an 2008 F53 with an 80 PSI recommended pressure you could have a DANA 80 axle as I do (same pressure recommendation) which is has a max weight of on 11,000 pounds. If you tire size is 19.5 you might be over weight. If 22.5 you should be OK but not by much. Now the bad news. Looking at your profile image it really looks like you have 10 bolt wheels. The DANA 145 axle Ford uses only has eight bolts.
HERE is the Ford published specifications for model year 2008 F53 chassis.
