I just bought a 1990 Multivan Pop-top.  I'd like any pointers about
> how to install an auxiallary battery under the drivers seat.

Jim,
I used the highly regarded Optima battery.... as far as I know it is the
only full sized (as opposed to the 'wheel chair' size) deep cycle marine
battery that will fit into the compartment under the driver's seat. The
Optima is also sealed and can be mounted on its side or even up side down.
Buy the one with side posts.

I cut two slots in the top forward lip of the auxiliary battery
compartment... one for each of the top posts to pass through.
And then two slots in the lid to assure the side posts don't touch the
lid... then taped over the two gaps in the lid with black duct type tape.

 Bill