Sounds like you are describing the transfer switch. If so, I replaced mine when it failed with this:
these people were good to deal with.
They also had Thin-Lite replacement ballasts which saved me quite a bit over replacing the failed 12 volt
DC fluorescent fixtures when I bought my MH.
You can also get the transfer switch from campingworld and other RV parts places.
If you want others to be happy, practice compassion.
If you want to be happy, practice compassion - The Dalai Lama
1984 310 Limited Motorhome
Courtesy Parking (W/S/E/Wi-Fi) on I-5 in Northern California, 70 miles from Oregon border