: to be clear, I haven't used the att home wireless service myself, but I have extensive experience with computer networking, a bit of coding, and also Verizon wireless (I service 500 retail locations) used as landline internet backup and it works quite well.
Hey weboughtabus, i'm planning a skoolie conversion and have asked myself the same questions. I'll be bringing my wife, a small dog and 2 cats so I want to be able to keep tabs on things while i'm away for a few hours, or possibly overnight.
I know in your original post you mentioned no wifi/hotspot but I thought i'd throw out an option that you may have not came across, and I just recently came across myself. AT&T offers a "home" wireless internet service now for $50 per month and comes with 170gb of data.
I confirmed with AT&T that it's not gps locked to a location, and specifically asked them if I could use it in a mobile RV and they said that would not be a problem and it would work perfectly. The only catch is that you have to use the provided hardware which isn't a huge downside. It's a little modem/router that looks like a regular home router.
I'm sure it likely uses an a/c power brick, but I would bet you could feed 12v straight into it, and it probably has a minimal power draw. If you intend on getting online a lot while in your bus this would serve multiple purposes at a resonable cost of around $50 per month.
With that being said, at that point there are a number of devices that could do what you are wanting to do, which you may have already researched. I think nest makes one that is temp only, and I would say there are several others. The plus to that is that you could have a well known app that has push notifications for alerts: IE nest app, etc.
Your idea of using a raspberry pi would certainly work, however I would say that a Pi is a bit overkill for this, and it has a lot of moving parts (software wise) that could break/go wrong. I've used a Pi for a lot of different uses and while they can be pretty reliable, sometimes they have their flaws, they crash, etc.
I would suggest an Arduino for something like this, with a temp sensor and a Arduino GSM shield (shields are modules for arduino) You would still need to write all the code yourself, then on top of that you'd need a way to receive alerts. You would still need a sim from a gsm carrier (att, tmobile) and the associated data/text plan. You can get these plans for pretty cheap, they call them m2m (machine to machine) plans.
Conceivably you could use an email/text message based system for sending alerts. If you used either of those you could also email or text the device a specific "code" and get a reply back as to what the current temp is. Pretty low tech that way, but you wouldn't need any "servers" or anything in the cloud to run it.