I was just thinking the other day.  How do tournament anglers charge their boat batteries when they are on the road fishing tournaments?  I'm assuming that they are staying at a hotel.  I know that boats have onbourd charging systems but I was just wondering where they plug them in at night.

We use an extention cord out the hotel door or window.

Some Motels even offer parking with locations to plug in your rig. Something to look for when booking a room

It is a little secret that the pros have.  What they do is invest in about 60 mice.  They have an immense charging system set up under the boat decks.  The mice sleep during the day and charge at night.

Easy, get you one of those windmill generators, rig it so you can mount it in the boat and when you head down the highway, that thing spinning in a 60 - 70 mph wind should charge them up pretty quick.

If you do a search, this subject has been brought up a number of times and you can write a book on the responses.  There are chargers, and Isolaters made for this purpose.  A couple said they have them and like them, I wouldn't waste the money or time.   Unless you're traveling between remote areas, there's just not many places than are not going to be able to provide AC power to run your charger.

Many vehicles that come from the factory with a 7 pin trailer connecter have one pin dedicated to a 14v charge directly from the alternator. Sometimes i use it to charge an extra "oh crap battery" in the boat while heading to the lake, ive also used it to charge my cars battery while its on the trailer on the way to the track. Now i dont know if id try it on a deep cycle with out some type of inverter ready mounted in the boat and hooked to the battery.

The charge from the alternator works just fine with camper batteries.  I charge mine that way when I'm in deer camp, my dad does the same with the two battery setup in his travel trailer.