I went with 4 golf cart batteries mounted under the seat next to the nav station which give me 490 amp hours and a 100 amp starting battery. There are several articles on the forum about this conversion

I have a 1984 Catalina 36 witch I just purchased and had a similar problem.  Most suggestions are from a time long past.  I first wanted Batteries that I could lift and handle myself.  So I settled on size 31 Marine Batteries,  Using that same position just forward of the Nav Station, I installed Three (3) 31's.  Completely filled the box with no room for movement, then strapped them down with a ratchet system.  My permanent Battery Charger from Catalina has three (3) outputs, so that is perfect.  Secondly I installed a Digital Battery Splitter in this box to separate the Alternator output, which charges the Battery that needs it the most, and divides the charge as required by the Batteries.  I ran a number 4 wire straight from my Alternator to this splitter so there is no need to worry about your main Battery switch position.  If you notice in the attached photo I also installed a second Battery switch so I can separate my house Batteries or put them in parallel ( Not something you want to do for long duration ).  I went with Lead Acid Batteries because of cost, but the Sky is the limit on expensive Batteries.