Based on feedback here, I decided it was time to replace the Parallax 7400 (55 watt) with the PowerMax Boondocker PM4B-60, before it felt the need to torch my batteries.
This isn't rocket science and I am not trying to "drop knowledge" on a forum whose participants are much more experienced than I am, but I do want to be an encouragement to the novice that they could take on a job like this when the need arises. However, everyone works on their own trailer at their own risk.
Let's get started. Following the instructions, ensure you are not plugged into shore power and disconnect the positive cables from both batteries.
Remove the lounge. On mine, it's five 3/8" self-tapping screws.
If you've got the vinyl flooring, gently move or slide the lounge out of the way without scratching the vinyl and giving yourself enough room to access the components.
Not pictured: Now vacuum all of the crumbs left over from the factory.
Uh oh. Houston, we have a problem. Never screw into the ends of soft plywood, especially without drilling pilot holes.
There you are. Loosen the four self-tapping screws, loosen each buss screw on the old converter, disconnect the wires, and remove the converter. (Then vacuum again.)
New solid wood bracing (with pilot holes drilled).
Ready to install.
Place the new converter in its new home. Connect the positive, negative and neutral wires in their respective buses, noting the polarity of each buss slot. Between the Parallax and PowerMax, the buses were reversed. It's not a direct "plug-n-play" in that respect. Then secure the converter to the floor. The self-tapping screws really come in handy as it is necessary to make new holes to secure the converter as the new doesn't match up with the old. Then plug the converter into the 120 VAC outlet.
Close-up of the connections.
Then vacuum again. Double check everything and reinstall the positive cables on the batteries. It's always a good idea to have a volt meter handy, but observing the proper connections, it's pretty hard to screw something up. Reinstall lounge.
Well that's about it. No major issues with the install except the shattered wooden brace, but that was quickly remedied with some scrap I had laying around.