I have used these instructions and they are quite good. He also sends detailed pictures that help quite a bit. I asked him several questions over a few day period and he got back to me promptly answered most of my problems. If you do not know how to do much electrical stuff, I would recommend his instructions. I have no affiliation with the seller.
     I never used the battery that he suggested as I do not buy anything from wal-mart and I "borrowed" my dad's spiral cell battery for nothing. I did run the microwave from my kitchen long enough to cook a potatoe but did not like how much room it took up so I took it out. I also went with a 700 watt inverter instead of a 1200watt as the battery will last longer and all I need are 700 watts. I have installed a solar panel to the side window behind the cabinet. This will allow my batteries to stay fully charged without having to run the engine.    
    It should take only half an hour to charge your battery depending on how low they go. The primary battery should not be drained from the second battery.
    I use the second battery to run everything in my van when the engine is off except the fridge. The fridge draws way too many amps and is better run on propane. Things I rin  on the second battery are radio, all lights, fan for hot summer nights , chainsaw, etc.
\Patrick 85 westy appin Ont