My guess is that the previous owner installed the battery tender because the batteries were not charging with the disconnects opened...
Another possibility: If the previous owner had to plug the coach into a GFCI outlet, and he was using a surge supressor on his shore power cord, the supressor may cause the GFCI to trigger. This happened with my coach in storage. Once I removed the surge supressor from the shore power cord, the power stays on reliably. The coach that is stored next to mine experienced the same problem, but his Surge supressor is built in.
If your batteries are charging with the coach plugged in, you really don't need the battery tender.
My storage just has a 15A power outlet. I use adapters to connect the 50A shore power cord to the 15A outlet. It's not enough power to run everything, but it's plenty to run the battery chargers and the refrigerators.
One tip... When storing, I keep all the adapters and the shore power cord in the coach power bay, and run a cheap grounded extension cord from the wall to the adapted shore power cord. The reason is that should I forget to disconnect the shore power before backing the coach, the worst thing I'll do is trash a $10 extension cord, not the 50A connection.
Hope this helps!