I would assume that these would be of more interest when
not hooked up to shore power, as they would run directly off of the trailer
battery power. This assumes, of course, that you like to read underwater... or that the
light pattern works out well in a more normal reading environment.
Then there's always
lighting the fountain that any really well-equipped camper packs along.
I think that a 5-watt incandescent bulb would need to be focussed relatively tightly to be bright enough to be a good reading
light, but at least the power consumption is low (for
battery use). Typically home lamps with low-voltage halogen bulbs use 20W and higher sizes.
As for the power supply: it could run several bulbs of this size, for various
lighting needs, but they could also run off the trailer converter. If the trailer doesn't have a AC-to-DC converter, then one large enough to run other stuff (such a vent fan) seems like a better idea to me.