A good quality battery charger is great for those times when you need a quick charge for when you left something on and killed a battery that you need for getting to work.
For charging a trailer battery at home when speed is not of the essence I would recommend a low power float charger... the "Battery tender" referred to by Lynmor.
My float charger with slowly charge a battery to full charge in about a week then maintain the full charge indefinitely. Slow charging a battery is better on the battery than a quick charge if you have the time for a slow charge.
I can leave a fully charged trailer battery in my trailer for months after which it will be about half charged. If your trailer is draining a battery in a week then you are leaving something on inside the trailer or there is something wrong... a bad battery or a large draw from the trailer of some sort. The salesman is telling you to disconnect the battery because he doesn't want to deal with the issue.