You should not need a transmission cooler. Your RV is built on a stripped Ford E350 chassis, and has a 18,500lb GCWR. When comparing that to the E350's GVWR of 12,500lbs, it should give you a theoretical 6,000lb towing capacity.
Other than the typical e-series suspension suggestions of adding stiffer sway bars and similar gadgets, one thing some motorhome owners that haul larger boats do is to add a front hitch receiver, specifically for putting the boat in the water.
With a longer boat, you might find you have back the RV too far into the launch ramp, and you might find you are getting the tail end of the motorhome wet.
I used to tow a 6,000lb 22ft cuddy cabin with a Dodge RAM 1500, and often the ramps were pretty shallow in my area, so much so that the rear tires usually were in the water. I can't imagine trying to launch that boat with my motorhome.
Putting a hitch receiver on the front usually solves that problem. It is usually not needed with smaller boats though.
The problem you may have though is finding a front trailer hitch. You can find them for a standard E series Class C motorhome, but a Class A, I am not sure if those are available.
You could probably have one custom made, or perhaps a commercial one for the van front end modified, but since that is a lot of trouble, you might want to try it first conventionally.
Some E series owners can feel the rear swaying when towing. I feel this with my coach when towing my 3,100lb vehicle. But it might be due to the 4 wheel orientation of the vehicle wherein the front wheels are steerable. Not sure if a 4 wheel boat trailer would have the same effect or not.
This is a commonly reported issue and a rear track bar (such as a Tiger Trak) seems to be the solution.
I am considering putting one on my rig as I can feel the sway at highway speeds. It is not a tilting sway (my sway bar upgrade fixed that), but rather a left-to-right sway. If I am not careful, I can get the car/motorhome to oscillate so that it wags between the centerline and edge of the road. Kind of like the tail wagging the dog.
But I think this is due to the steerable front wheels in my tow vehicle. I am thinking that a boat trailer might not do that, so I would not consider a trak bar in your situation until I was sure it was needed.
The only thing that works on a RV is the owner...