Weather you *need* it depends on a lot on the trailer's construction.
My TT is 38' overall with 4 slides. It's never had trouble cooling down quickly, with a single 15K BTU Coleman, even when I forgot to turn it on before leaving. My former 41' weekend warrior (with a single small slide) performed almost as well.
The trailer before that was a 29' Jayco, no slides, with a 13.5k BTU unit. It could barely keep up, and if you didn't keep it on, it would never cool things down.
That said... I do wish my trailer had 50-amp service, so that I could run the Hot Tub without a 2nd cord.