Do all restroom trailers need a fresh water hookup for a 1-day event?

Yes, a restroom trailer can run a one day wedding on its onboard tank, as long as the rental company sizes the tank and stall count for your crowd instead of you guessing. Most of the trailers I've had at receptions in fields with no spigot ran the whole night on the fresh water tank with no problem.

The thing that actually caught us once was the other side. The holding tank filled up before the fresh water ran low, and an open bar is exactly what pushes that. So when you call them back, ask three things: the rated number of uses for that trailer on a full fresh tank, the rated number before the waste tank needs pumping, and whether they will top off or pump on site if you get close.

More stalls usually means bigger tanks, but don't assume it. Ask for the numbers on the exact trailer they are sending.

Also confirm power. The sinks, lights and pumps on most trailers need a hookup or a generator, and a grassy field usually means generator. Have it fueled for the full event plus setup and teardown.

Last tip: have them fill the fresh tank at their yard and deliver it full. Planning to fill it on site with no spigot nearby is how you end up with dry sinks at the cake cutting.