Thanks for the suggestions, guys. I exhausted all available NEMA options fairly quickly, so you are all correct that this is [most likely] a "non-NEMA" Hubbell-specific inlet.
On idroba's link to the NEMA chart, I guess this is identified as a 3-pole, 4-wire grounding plug. All those listed are either 50-amp or 60-amp plugs!!
This trailer actually already has 2 air conditioners in it, but honestly, I'd prefer the smallest (5,000 BTU) unit I could find and let it run all day. I likely won't go removing either one, however, because I don't have the window to put back in. Not sure why the previous owners had to put the 2nd air conditioner in and remove the window.
It's more efficient to get a small a/c and leave it on max cool every minute of the day, than to let the compressor cycle on and off. (See people using ~800 W generators from Harbor Freight to power their 5,000 BTU window units!) Just removing the humidity is all I need to feel comfortable.
It's got an electric fridge/freezer that's maybe 7 or 8 cu. ft. I've already installed a 2 cu. ft. microwave / convection oven (before ever testing the electrical hookups). And the only other heavy-consumption electronics I'm thinking of installing is either a single- or double-burner induction stove. The other stuff would all be efficient -- LED TV, laptop, cell phones, etc.
Soo.... even with just one air conditioner, would this seem like enough for a 50-amp inlet??
Is there a way to convert Wattage into the proper amperage rating that I'd need?? I have a Kill-a-Watt device to figure most of this out tomorrow.