Line the bottom of the trailer with sand and wet it. Avoid routes where you will be sitting in traffic. A trailer standing still will heat up very quickly in the sun without air moving through it. Use air conditioning to keep them cooler if it is available in the vehicle. Put a thermometer in the trailer and keep track of the temperature.
Cold temperatures are much less of a problem unless they are extreme.
The windows and vents on the trailer should be closed to avoid wind chill and frostbitten ears. Keep a thermometer in the trailer to be sure that temperatures do not get too cold. If there is a cria in the vehicle or a llama with very little wool, consider putting a blanket or coat on them to maintain their warmth. Halters can be a problem during transportation. Llamas are very mobile within any vehicle that gives them room the stand up and lay down.