Our camper came with one factory direct


I agree having one is very convenient. While the wing nut on the negative for sure works, I myself would not want to have to deal with the evil battery case straps.
If you add one, make sure it is rated heavy enough for the amp draw. And make sure you wire in the emergency break away switch hot all the time, upstream of the disconnect. You really do not want to accidentally shut that feature off.
The comment about the disconnect not shutting everything off inside the camper may be a case by case basis. Mine wired on the hot lead from the battery does kill everything coming from the battery, other than the break away switch and my power tongue jack.
Blue Sea Systems is the manufacture of mine. They have several types.
http://www.bluesea.com/articles/91 They are sold on Amazon and many other places. My pictures are from a 2004 camper, Blue Sea changed the looks since.
Hope this helps
John