If you look at the sides of a trailer and there are miniscule cracks on the side walls, you should replace the tires. There may be great tread, but that doesn't mean they will last any time at all.
That is good advice on the tires. I had two blow outs on my horse trailer because of dry rot. I knew I needed to replace them but didn't have time luckily I had some spare tires to throw on there until I got them replaced. If you have the trailer set up alot spray the tires with armorall every couple weeks and let it soak in. It will help keep the tires from dry rotting.