maintaining house batteries with a trickle charger

I have a 26 ft class a rv . I rarely stop for more than 3 days but always "dry camp " but do sometimes get a low battery after 2 days . Would a trickle charger keep this from happening ? How much power would I need ? Price range? Does a trickle charger help to prolong battery life since the Rv sometimes sits idle for months ? I currently turn on my generator once a month to recharge the 2 house batteries .