Whats the right way to hook up a power invertor and 2nd battery in my ice cream van?

brought an old ice cream van the guy i brought it from said he had an power inverter and 2nd battery hook up in it before. so i went and got a new power invertor and extra battery (12 volts) and put it in the van. Put the battery in where he had his battery before wires where already ran and i mounted the inverter on the wall and ran battery cables from the battery to the inverter. worked that was monday. Yesterday i got in the van and it would not start had to get a jump ran it for awhile cut it off battery under the hood dead. i havent even ran the inverter only turn it on when i first hooked it up to make sure it worked.