Battery question in conversion camper

I would like to get agm batteries but the wife wont let me spend $450 on batteries. I found duracell 6 volt batteries i can get. 2 for $255. Is it possible to build a battery box with two vent holes going to the outside. Make it out of osb and find a type of paint that can withstand any acid spray. Caulk all the seems and use a type of rubber or foam around the door on the box. Ive always camped in a rent but since my baby came along i wanted to make it a little more comfortable. I was looking at converters. Found on etrailers a ctek 7002 ac to dc converter. Has anybody used that one or any ctek converter.