I just installed mine Saturday. I used a scaffold at work. It took me longer to set up the scaffold and put down some roof protection than it did to do the job. I put some blankets on the roof and a couple pieces of plywood I had. A co-worker came by to help me with the lifting.
I got mine unit from PPL. I had it shipped to work, instead of home, and saved around $40 on the frieght.
https://www.pplmotorhomes.com/parts/rv-air-conditioners/rv-air-conditioner.htmCheck your model number on the old unit and cross reference to make sure you get the right one. You can buy the roof unit alone or both the roof and ceiling unit together.
I found a deal locally on a Dometic unit, but couldn't use it. My unit has the controls in the ceiling unit, some brands use a thermostat that is wall mounted. It wasn't worth modifing to change brands.