Somewhere the mode switch triggers an electrical switch (either directly or by vacuum) that is probably energizing the compressor clutch and the condenser fan (+- 5 amps), the latter being a 15 to 20 amp draw. Acme, who makes mine, told me that on Diesel coaches the compressor runs mostly all of the time except in floor heat and guess what?----vent. He said that unlike a gas engine, you don't feel any power loss from the compressor (they have changed that on newer coaches). While I have to agree with him on that, it does put wear and tear on the compressor and the condenser fan. The alternator has to work harder, and it does waste fuel as well. The system cycles the compressor by using a "freeze switch/t-stat" in the evaporater. I put a compressor switch on mine to save the wear and tear and hated seeing the ammeter drop down too. If you have a compressor switch, shut it off after you see the big amp draw and I'll bet the ammeter comes back up.
Good luck, hope this helps,