Power draw on 04 American Tradition

[2004 Gypsy]

Hi all, we recently bought a 2004 American Tradition 40j MoHo from an older friend. He kept it immaculate and always stored in a shed when not camping. It has around 35,000 miles on it and looks like new. He said in its day it was the top of the line. At any rate we paid $75,000 for it; and think we got a pretty good deal. We had it set up in a camp ground up here in northern Wisconsin for the summer and have noticed some things that need fixing. The main thing is when plugged in I can hear the inventor fan running every 15 minutes even if we aren't using much power. Fridge and water heater are on A/C. Not using the air conditioning or micro wave. This happens constantly. If I don't leave plugged in when we go to RV the following weekend; batteries are very low. Im thinking we have bad batteries and I'm going to switch over to lithium batteries and Victron inverter. I took out the old deep cycle batteries and put in 1 good AGM truck battery I had laying around just so I have a battery in it so I can slide in and out getting it ready to go into storage for the winter.When I connect the battery I see sparks and hear a clunk from a box at rear of battery compartment. Seeing sparks tells me I have a draw. Is this normal to have this draw ? is it just a solenoid clicking to battery power and stops drawing after it switches ? I have everything I know of turned off and un plugged while doing this.

Thanx in advance...Barry