The American Boat and Yacht Council (ABYC) is pretty clear on the issue of solid wire. It breaks when subjected to vibration. And it isn't the amount of vibration, it is the issue of long-term vibration.
Boats are subject to a lot more vibration and extreme movement than travel trailers. But travel trailers and RVs are subject to more vibration than your average house.
Practical Sailor summarized some of the main points in this article.
(It is a PDF. You have been warned.)
The conductors used in marine applications
must be made of stranded copper wire. The black,
white and green ground conductor in each cable or
cord must each be of the stranded wire variety.
Household conductors, on the other hand, have a
single, solid core wire. The number of strands
required in each conductor in marine applications
is directly related to the conductor’s diameter
and the degree to which the wire will be subjected
to flexing through movement or vibration.
The conductor’s proper size and diameter is also
determined by the demand or amount of current
the conductor must carry.
I know a lot of folks like conduit, me included, but you have to be sure that you don't trap water. Conduit isn't recommended for use on boats.
Good household wiring often travels through
the house in metallic tubing called conduit. The
conduit supports and protects the wires within
the walls and ceiling of your home. But on your
boat, conductors are supported throughout their
length by a self-draining loom or are secured
every eighteen inches by straps or clamps.
It has been my experience, that you always want to provide a route for water to drain away. You probably can't keep all the leaks out... give any water a path that does the least harm.
"Houses are but badly built boats so firmly aground that you cannot think of moving them.... The desire to build a house is the tired wish of a man content thenceforward with a single anchorage. The desire to build a boat is the desire of youth, unwilling yet to accept the idea of a final anchorage." - Arthur Ransome: Racundra's First Cruise, 1923