No need to change the oil. It's the synthetic hfc refrigerants that aren't compatible with mineral oil.
I would look for a larger unit to modify, though. A 5kbtu compressor might only move 3kbtu after switching from 410a to 290. You might be surprised how much heat leaks out of rv's and campers when it's cold outside. I would really be tempted to hack the rooftop unit myself. Especially if it was out of warranty.
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