Two options depending upon how far away from the RV you wish to be. If within a few hundred feet, then something like this could work for you:
Remote temp sensing from Amazon.
If you want to be miles away and know the temperature, then you need a way of transmitting the data from the RV to your phone or laptop. If you have always-on WiFi in your RV (like a Verizon MiFi for example), then this is an example of the type of device you could use:
Internet based remote monitoringIf you don't have always-on WiFi, then things get expensive. You would need a sensor that can connect to the Cellular Network. For that, you need an access fee. While Verizon does have a $5/mo plan for machine-to-machine connections, it may be hard to get this as a private individual. Then you're looking at data plans on the order of $50 per month. Here is an example:
Cellular sensorI have the first one on order from Amazon. The other two I have looked at, but I don't have any actual experience with any of these.