Think of it like this: Sure a trailer's spinning wheels can splash water up into the hole, but the point of having the hole there is to allow water to flow back out. Based on the very essence of a favorite physicist (Newton) and popular Blood Sweat and Tears rock song "What goes up, must come down. Spinning wheels got to go round." ;-)
As for the ants . . . they can squeeze in through any number of small gaps and spaces, so short of putting a dab of insecticide around the tires
and hookup hoses and cords there's no keeping them out. If you do get ants at some point simply moving the trailer from one place to another will generally end the infestation.
What you really don't want in your trailer is a place ants might like as a nesting spot. What they look for is wide (by ant standards) very narrow space between two surfaces where they can build easily protected nursery colonies. The big, open spaces of the pontoons don't really qualify.
Kudos to Reace for improving on the Trillium
design by making the "pontoon" gutters deeper and providing more vent holes. Our Trillium-inspired Surfside
project tralier has a rotten floor problem because the gutters were not quite deep enough and only one vent hole was drilled at either end, so when I came to pick the old maid up and tilted her on her axle
a lot of water poured out.
The whole floor may have to be replaced.