When I got my Trailmann Trailer new in 97 , I used a broom and mopped used motor oil on the pressure treated wood floor one hot summer day.
Repeated it every year up till about 2004 ... then coated the floor with black tar ( found at Tractor Supply basically foundation waterprofing Tar )
Kind of regret the Tar. Still a littly sticky on a hot day, upsets the wife when shoes have a little tar on them and come into the house
If I would do it over with the new trailer ... hindsight is 20-20 of course ... I would go to Lowes and pick up some 3/4 inch thich , 4x8 sheets of Pressure treated CCA Plywood and cut them to size , and use stainless steel deck screws to secure to the Pressure treated floor. Changing the Plywood after many years would be simple compared to replacing the original floor.
At far as a nonslip , it's hard to beat a Horse panel ( 2 in x 4 in spacing ) secured to the floor ( available from Orscheln's Farm and Home) .