Objective: House Battery

So here is my quandary..  I'd like a house battery.  I'd like to set it up to be charging from the alternator while driving, but then be isolated when the ignition is off so as to not draw juice from the starting battery.  A continuous duty solenoid would accomplish that.

Now, here is my comparisons.  I have my Van Halen..  which is a 1996 Ford E150.  Here is a front view of the motor:

Here is a view to the left of the motor, and the space that is there:

Have been told that space is there for an extra battery for if this was a diesel motor.   Why couldn't a deep cycle house battery be put there?

It should.  Here's why I think it should...    Right now, my RV that is up for sale is a 1995 Ford E350.  I took comparison pictures, here they are:

Same placement.. starting battery on right and the house battery is in the same spot my empty space is on the van, here is the comparing shots showing the house battery:

Tell me why that wouldn't work?    It should!   Now..   just to find out who could wire such a thing...   Suggestions anyone?