Battery Charging and VSR's

I currently have a late model Isuzu Dmax which is about 2 years old. Whilst driving from Darwin to Brisbane I noticed the battery was struggling some mornings. I decided to replace at Battery World (fantastic service at Indooroopilly btw).
I was a bit surprised that the original battery didn't last too long. Essentially, the tug gets driven about 5-10 km a day during the week, with a few camping trips and one long road trip a year. Hence, it's never really getting charged.

A mate suggested a battery charger and hook it up once in a while, which I will get.
I have a Century Ultra high Performance NS70LX under the bonnet for starting, and a Century Marine Pro 730 in the tray. Both connected by a Redarc Voltage Sensitive relay (VSR).

Two questions - Is it ok to hook the battery charger up to either battery as I am not sure that the VSR will interfere (for want of a better word) with the charging, or do I need to disconnect a battery before charging it?

Second - I am thinking along the lines of a Ctek MXS 15 or a Projecta 12v 15amp job. I am happy to leave it on charge over night, so it doesn't need to be recharging at a fast rate. Whilst at it, I may also use it to charge my Century Battery in the Toyota Aurion.
With batteries expensive I'm not in a hurry to replace another one any time soon.

All responses appreciated in advance. I'm not too much of a tech head so all advice welcome.