There are lots of ways to get an A/C unit up to roof level. I now work alone and change out a roof unit on MoHos and trailers about once a week! I have used 4 methods, and they have evolved accordingly.
1. Using a straight ladder, lean it against the trailer with a well padded 2 X 6 in between the ladder and trailer. With the unit still in the box, wrap a heavy nylon strap around the box and place it at the base of the ladder. With one person (me) on the roof with strap in hand, I start to lift the unit in the box (usually 120 lbs) while a helper starts to lift from the bottom, eventually pushing it as you pull. The entire job takes less than a minute!!
2. Go to your local equipment rental place and rent an air conditioner lift. It will easily hanadle your A/C unit, and if you are on pavement, will easily roll under the trailer, placing the unit within easy reach for you to move it from the lift to the trailer roof. They usually rent for about $100. This works well, but the rental place was very unreliable with delivery for my tight schedule, so I had to go with #3.
3. Using method #1 wiith no one else around, secure the ladder in place with straps so it doesn't slide into the trailer and lift it up the incline by yourself (I do this now on a regular basis) using a hand-over-hand grip on the strap and a set of heavy gloves.
4. I now use a modified method #1, but have invested $125 for a 120VAC winch motor with 220lb. capacity. Had a set of extensions made from aluminum tubing that fits the top of the ladder, and now I stand on the roof with a remote in my hand and wait for the winch to raise the unit to the roof. I just have to manuver it off the incline and the rest is easy!
Feel free to PM me if you need any other info..............................
PS: the A/C manufacturers give me only 1 HOUR!!!! to remove an old unit and replace it with a new one, wiring included, when doing a warranty replacement. I've got it down to about 50 minutes!!!