Haven't tried that solar vent, but the powered vent retrofit kit from ETI is very nice if you haven't seen it. It uses the Ventline Vanair fan. If you want to use the solar vent you are going to have trouble mating it to the existing 9-1/2" square opening without some sort of adapter. If you insist on using the solar vent maybe you can get the fiberglass adapter plates from ETI from the fan kit (see pic) and increase the diameter to suit your needs? Could probably make it look pretty clean and get it nice and watertight.
Solar vents typically do not move air quickly. Their benefit is moving low volumes of air constantly so that it adds up. I think they are a good addition to any trailer as a complement to powered ventilation.