Lancer, I still think you should take it to a scale next time you are out. Only costs $10 here and well worth it.
D rated total 10160 and you are towing a 36' trailer, 8500-9500#. and $155k boat. I would prefer the peace of mind knowing what the actual weight is and I would add the extra axle for safety.
You are very lucky the loaner tires didn't blow and cause who knows what damage.