You should be able to get on the roof, stay away from the ends, they have a tendency to be a bit weak, especially the front. I use a small extension ladder placed against my rolled up awning for access.
As far as parts, good luck. You can purchase replacement covers. But for replacement parts, highly unlikely. Most RV a/c units are designed to be replaced, not repaired. They don't even have refrigerant ports on them anymore.
Aaron
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