Having a professional do it will get expensive.
Do-it-yourself requires some knowledge and a good machine, and will probably be less than perfect on your first try.
My mom signed up for an upholstery class at a community college to do her RV. Expert help along the way & access to commercial machines gave excellent results. It took her two semesters to do everything in a class C motorhome.
Or just leave the brown cushions and dress the trailer up with accessories as others have suggested: pillows, curtains (much easier than cushion covers), throw rugs,
paint cabinet doors, etc.
Just don't let renovations interfere with using the trailer!