Winona,
We had the same concern leaving our dogs in the coach while we were away on a hot day.
We ended up using a Canary All-in-One Home Security Device with a wireless hotspot. It sends temperature, humidity & air quality alerts to your phone if one of the settings you set is exceeded. It also gives us the look-in live if we want via the HD camera. It uses very little data on your plan... unless you use the video option a lot. As an added benefit it's a security system as well when we're not on board.
The other system I looked at was the MarCELL 500 Cellular monitoring system. It seemed to be a great option if wi-fi isn't available as it uses cellular and is self contained. The downside is that it requires a small monthly cellular charge.
Hope that helps!