Battery Change in my Motorhome

I have a Chausson Welcome 78 EB motorhome with two batteries. One for the vehicle and the other for the habitat module, The habitat battery is a 80 Ah. I am considering buying a 120 Ah battery as a spare for use when we go on sites with no hook up. Are there any problems with the on board battery charging system that I need to be aware of or any other issues that I need to look out for?

Thanks for any assistance