Update:
Got an email from Adventurer and they said it was the base for a Jensen OMNI TV antenna.
Found this diagram below that confirms that.
I think what happened the previous owner must have removed the Jensen antenna and installed a different brand in another location but with the antenna gone they failed to seal around where the cable enters.
Luckily it has been an extremely dry summer. Camper was stored indoors for 2 years before that. So I will confirm this and seal it when I remove the tarp.