salem 402 2b lights flicker dim then bright

[ecrissman]

We have a 2008 Salem 402-2b PT, the camper is parked year long at a camp ground and alway connected to 50amp electric except winter. Last year late in the fall the lights would go dim then bright. The more lights you turn on they stay bright, you can hear the converter fan running. When only a couple lights are on the lights are dim.
The battery was bone dry last year, I added water and still same symptons. I removed the battery this last weekend (no suprise) same symptons. Other than the lights everything else works perfectly.
Seems obvious I should replace the battery, but I didnt think it was needed when connected to electric. Either way I will buy a new deep cell battery this week.

Does the battery need to be connected to "buffer" the electrical connection?

Any other suggestions, experiences or advice?

Thanks for taking the time to read this.
Eric