One persons take on the battery issue.

When a lead acid battery is recharged, it produces hydrogen gas which is very explosive if ignited with just a spark. The gas "hangs in the enclosed areas". I worked in a new car dealership for seven years doing tune up and electrical repairs, and was a service manager in a motorsports dealership that handled thousands of lead batteries every year.  Fortunately, I have seen only one battery explode in the past fifty years of playing with Hot Rods/ski boats/Jet Skis/ ATV's/motorcycles/race cars/travel trailers/etc. In that case (it occurred about 1968) we had a very large heavy duty battery mounted inside the rear trunk of a drag race car for weight transfer. We had the battery charger turned up on high with a heavy charge going into the battery, producing lots of gas, and in a very confined area. When we disconnected the battery charger, the battery exploded because the hydrogen gas was trapped in an area under the cars inner trunk/fender. This could have been corrected by turning the charger off before disconnecting the cables. It would also be a good idea to use an air hose to gently blow/vent the fumes out of an enclosed area before removing the battery.

I use small 2.5MA battery maintainers to recharge my 12V battery in the travel trailers. I have never seen, or smelled any excess fumes produced by these small chargers. I'm sure they produce fumes, but it's just a very small amount.  These "full time" small maintainers do a great job of keeping the battery charged, they just take a longer time to get up to full charge.

I use a separate fuse panel, and also a separate ground panel for trailer wiring.  The battery is "hard wired" from the positive, and negative terminals directly to these two panels.  The positive cable has a circuit breaker in line.  Any time that its necessary to manually disconnect the battery, simply do so "from the fuse panel first" and any spark that might occur will not be near the battery, or near any "possible hydrogen fumes".  I feel the dangers can be safely managed, as long as you understand how, and why a battery might explode.  It's about how you work around them.

Acid Glass Matt (AGM) batteries have the same hydrogen gas problem, but they are not as good as a normal deep cycle lead acid battery for our purposes.  IMO, the dry cell batteries are over priced, and over rated, but they do not "off gas", so that's a good thing. 

I'm very hesitant to give any electrical advice, because we have so many different levels of knowledge on this site.  It's very important when working with anything safety related that the person doing the work have a good basic understanding of what can happen if not done correctly.  Welding, Suspension, Brakes, Electrical, Tires, Propane, are all areas that require a good basic knowledge before you start slicing and dicing.