First: Give the trailer an air ride suspension.
Second: Automatic air-conditioning and heating system. Can't let the fish get to hot or cold. That could be suboptimal or even seriously suboptimal.
Third: Make a sturdy fish tank that has a lid that seals, but has splash proof venting for air, and plumbing for filters. It also needs to be able to be firmly attached to the trailer.
Forth: Make some fixed tank furniture, and bottom rock bed. That way it doesn't move when the water sloshes around.
Fifth: Use an external sealed canister type filter for cleaning the water. Yes, these are a standard fish tank filter design, or at least they were back in the '80s.
Sixth: Pick some hearty fish. Ones that like swift water may be the best choice.
7th: I'm sure there is more, but I'm not going to bother to figure it out.