One of the two deep-cycle RV batteries on the travel trailer had suffered a "low-water catastrophic failure event"
In other words, because we'd forgotten to top up the batteries with distilled water for a while, the battery had exploded! As you can see, the battery on the right blew part of the battery casing off. The battery on the left has its caps off while Lisa refilled it. It still works, but both batteries were eight years old and were well overdue for replacement.
In this view, you can see down inside the battery looking at the acid-covered plates. Lisa figures that what we heard was a hydrogen explosion. While there was some splatter (you can see some of it on the body of the battery) the battery box contained some of it, and Lisa quickly cleaned up the acid splatter on the body of the trailer and surrounding area with a water-and-baking-soda solution.
I carefully lifted the damaged battery out of its bracket and into a battery storage/transport box, and Lisa (having powered down the trailer when she realized what had happened) tested to make sure that the other battery was still providing some power. While the trailer is on shore power, all of the 12V appliances draw from the batteries while the batteries are charged from the incoming utility power.
With the immediate problems out of the way, Lisa could relax. That bang was much louder to her than it was to me, and it understandably rattled her a bit. With one battery still working, we did not have to immediately rush out and buy a new one. I checked to see who sells deep-cycle RV batteries in the area. Our past experience with such purchases told us that the shops in Fernley only tend to carry one in stock, and Lisa also didn't want to get an off-brand battery, given how hard she works them.
Today, when I got off work, we loaded the failed battery (Lisa snapped the piece that blew off back onto the battery) and the nearly-dead "hotel" battery from the Big Orange Van into battery-transport boxes and put them in the Astro. We then headed into Sparks to the nearest Interstate Battery dealer, which is a boat-and-marine-supply store. I'd called ahead to confirm that they had at least two in stock. We turned in the old batteries for the core charge and bought a pair of new batteries, including the one in the picture above. Interstate used to have a 60 month warranty on these batteries, but now it's only 12. Still, we got nearly twice the expected life out of the battery. We will later take the old remaining battery from the travel trailer in to the dealer and buy a third battery to replace the "hotel" battery in the Big Orange van.
The new batteries are unloaded next to the trailer and Lisa will install them at her convenience. This was a bit scary at the start, what with the battery explosion, but it all worked out okay in the end.