[shocker]

Before you can make a good decision on a tow vehicle, you need to know what you are pulling.

Go to a truck stop and weigh your trailer on a scale. Weigh with the trailer attached and then again with just the tow vehicle.
First of all this will tell you if you are overloading the tow vehicle and/or the trailer.

From the weighing, you can easily determine the actual payload the tow vehicle rear axle is seeing and how much the tow vehicle is pulling.
I personally want at minimum a 10% cushion between what the tow vehicle load specs are and the load.
Also, make sure your 5th wheel hitch meets that trailer weight.