Find a tent trailer that is 3 to 4 years old. This way it will still be in good condition but will have depreciated in value enough to make for a great deal.
Inspect the tent canvas for any signs of mold or mildew. This is probably the most important thing to look for and can be done by simply smelling the trailer when you step inside. Even if you don't smell anything, be sure to check visually as well.
Test all of the appliances and make sure that they are in proper working order. Run the sink for a while and flush the toilet a few times. The last thing you want is to have to pay for repairs on appliances later down the road.
Check underneath the trailer for any leaks. Obviously any leaking is never a good sign.
Inspect the body of the trailer for any serious rusting. This is also a red flag because major rusting indicates that the owners have not taken proper care of the unit.
Check the tread on the tires. There should not be an excessive level of wear or uneven wear on the tires. Uneven wear means that there are problems with the axle.
Shop around before you make a decision. It's best to take your time when making a big purchase such as this, so make sure that you find the right tent trailer for you.