To seal it use butyl tape all on the surfaces that contact the wall. Get the thinnest you can (1/8” or less). It’s the same stuff they seal the windows with. Then caulk the top of both mounting flanges (may be tough to get into the bottom one).
However, the larger issue is the structure its mounted to. Given its size and mounting holes I suspect you may only hit one ‘stud’, which may be enough. In that case only the center holes are used lessening the potential for water ingress. But if not strong enough to support the weight, consider a couple of strips of ¼ thick aluminum flat stock (available from Home Depot). Mount them horizontally and cut long enough to span 2 studs. Again, use butyl tape. Drill and tap them (prior to mounting) for the mounting holes of the TV base, centered between the studs. You may have to cut the bolts since they will be short. And if leaving the base attached to the rails, use Loctite to preclude having it fall off while on the road.
Good luck and let us know how it goes!