My wife and I (and Maddie) are very aware of the dangers of heat in a trailer. We have put together some safeguards to help when we leave Maddie inside the trailer.
We bought a Tempstick for our trailer and our truck. They are about $150 and by far the best wifi temperature and humidity monitor I've found. It is very reliable. You need to pair it with a hotspot for your trailer. Please do not rely on the campground wifi. It is not reliable enough.
The Tempstick can report on intervals the time, temp and humidity inside the trailer. We usually set it for every 15 minutes. If the temp goes above or below a preset number (say 78 and 45), we have it programmed to text us every 5 minutes. If you don't get a text, its time to call or return to the trailer. So all along we get 15-minute texts and then 5 minutes if the temp hits an alarm setting.
When Maddie comes with us in the f-250 diesel, and she is staying in the truck for a short time (less than 20 minutes, we leave the truck running (make sure you shut off the auto stop that is set for 25 minutes). We bring the temp stick and hotspot and have it send us info every 5 minutes. We check on the truck also by physically going to visit Maddie for a treat.
Be very careful with your animals! Spend some money on good tools to help monitor the situation when you just have no choice but to leave your friend.