Stranded wire is not insulated the same as Romex.
Stranded wire can be used, but it needs to be in conduit for protection.
It can be harder to get a good connection on a device.
While there is merit to the thought that the stiffer solid core wire could work harden due to vibration or movement, I would be very worried if it were a serious consideration in a trailer?
A cable connected to a motor or equipment that moves or vibrates
should be a very flexible stranded, but in a tear/trailer it should make no difference.
If it does, there is something very wrong with the suspension, or with the installation of the wire.
RV's go off the end of the assembly line every day with Romex...
You often have room in a typical electrical box for a splice, but if you need extra room and don't want a double triple, or more, one of these "old work" boxes will give the room you want while maintaining the look as the extra portion of the box stays hidden in the wall.
Rob

Waiting for "someday" will leave you on your deathbed wondering why you didn't just rearrange your priorities and enjoy the time you had, instead of waiting for a "better" time to come along...