op says wrote:
One note, I am trying to stay with 15" wheels because I have two spares I carry on my long trips. I am trying to avoid buying 6 tires and wheels so that is why I am "pushing" a little harder for a 15" tire choice.
Your 15" spare tires work fine on a trailer with 16" tires and wheels. The spare generally is on the ground just to get you to the next tire center anyway. Now if your the type that buys 5 tires and rotates them then stick with the same size/tread type.
I've bought three different 5ers over the years that came OEM with 15" ST225/75-15 D or H78-15 D tires. I sell them and go with a 16" LT E but I keep the 15" spare.
Tires on a trailer are more forgiving to a extent using different sizes/load ranges than the tow vehicle. Biggest issue from doing so is braking (tread types) especially on wet pavement.
Avoid using tires with the large tread blocks and voids like the mud tires or rougher all terrains unless your trailer is used off road.
IMO FastEagle may mean the Wrangler HT which is a very popular tire for trailer users of all types (including RVs) with 6k axles and smaller.