Recommendations?
Try before you buy. Rent something the size you are thinking of purchasing. Find out before that you need more space/seating.
Composting toilet? For a family of 5-6, not gonna work. Overload.
For anything longer than a couple of days, a spare gas ring (turkey burner) is wonderful and keeps from heating the entire trailer up in summer.
Auxiliary power cable and alternative gas source.
It may be just a regional SHTF and you end up parked at cousin Jethro’s place two states away, riding out the hurricane.
Being able to piggyback off his houses propane tank, water and power makes your life so much easier.
Don’t max out the vehicles tow weight with just the trailer alone. Once you start adding gear, food and water, you can quickly overload your tow vehicle. Running a second vehicle loaded with food and water helps immensely.
Trailers commonly need seal work done every 5 years or so, plan on it as a regular chore.
Buying a used trailer, I would plan on it before the next winter/storm season. Black mold and water damaged wood are no fun.
All of the normal maintenance items also apply, grease the hubs, the ball, use antifreeze in the water system, and so on.
Right now, I am considering the rebuild route, picking up a $500-1000 trailer and doing a gut and rebuild to it, to get what we want.