How critical is it to have a perfectly level trailer?

Another reason to go LiFePo4 battery is NO outgassing, so you can mount it anywhere and no liquids so it can be mounted in any orientation.
My emergency trailer brake power is supplied off a bolt on a buss bar exposed behind the front frame crossmember. It’s also where the primary battery cables connect to the trailer 12vdc system. I cut a hole in the floor there and rerouted the existing supply cables into the storage area to remount my battery inside.