The electrical gurus will probably chime in to answer this better, and in more detail that I can.. but the 2 have very little to do with each other.
Your rig has 30-amp service for the entire rig. But there are various different circuits in your rig... and the circuit that at least one of these appliances is on is only rated for 15 amps, and something is overloading that.
Now I doubt both the AC and the fridge are both running on this circuit simultaneously, unless they may have been possibly wired wrong from the factory. Is this rig new.. and if so, has it always done that since purchase?
If not, then either something else has got to be running on the same circuit, or... the start-up power necessary to get the AC compressor going is probably the culprit. AC compressors require a big draw to start up, but then don't require as much to continue running. To test this theory... turn off the fridge, start the AC up & get it running... then power-up your fridge again. If both continue to run, then that may be the issue... which can possibly be solved by installing a "soft-start" kit on the AC. But I'd get a 2nd (or 3rd) opinion on that before investing in it.
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