I found that most proper leisure batteries were too big to fit under the seat - especially if you want a 110Ah battery.
On my 51 plate SWB I put the leisure battery against the offside rear wheel arch - as I had cupboards fitted down the offside of the van, this hides it away but leaves it accessible when necessary.It also means the battery is quite close to my Zig unit and inverter, which are mounted on the cupboard wall above and in front of the battery.
For charging while on the move I just had to run a single thick wire up to the vehicle battery (split charge relay or go for the ultimate while-you-drive solution with a Sterling battery to battery charger!).
I did have to vent it - so in went the drill through my insulated floor and eventually out the bottom of the van. I put a bit of Sikaflex around the vent pipe where it comes through the underside of the van and everything appears to be ok so far. I used plastic tubing from my wife's plant watering system - but apparently windscreen washer tubing is ideal too.
Check where your drill will come out before you start drilling...
I used some L-shaped brackets to hold the battery in place against the wheel arch and a battery bracket to hold it down to the floor.