The easiest location would probably be where the factory installed them. The location changes depending on the model and year.
Mine has a Sears 5,000 BTU unit that sells for around $100. It replaced the original that was damaged in an accident. Some trailers have the ac unit behind a cabinet. Due to that an ac unit with a sloped top that blows upward works best.
Give Columbia Northwest a call. They sell an install kit that will have everything you need. They usually don't reply to email. I think others have said the kit was around $100.