Shelley, driving through Mexico can be risky unless you take proper precautions. Firstly, it's not recommended you drive at night - that's when the livestock wanders onto the road, the potholes are invisible, the drunks are around and the bad guys are active.
There are certain roads and regions best avoided because of the drug gang activity. Not that tourists are being targeted but you could be in the wrong place at the wrong time. Here's the worldnomads.com article on US state departments advice on Mexico travel http://journals.worldnomads.com/safetyhub/story/82854/Mexico/Mexico-Travel-Alert
it lists those roads and highways to avoid.
Obviously you'll be using the Pan American highway, which generally is good and relatively safe (for the cautious). I've spoken to septuagenarians who do it in their RVs, and 20-somethings who've done it on motorcycle.
In Guatemala be aware that break-ins of cars is pretty common, so don't leave anything valuable in the vehicle.
Make sure you've got good travel insurance that has the right level of cover. Enjoy the adventure and let us all know how it goes.
Phil from the safety hub. about 5 years ago
I did that drive just a few months ago and it was perfectly fine. No military stops or other problems. The road is currently under construction though so there are parts where it becomes one lane or is a dirt path detour. I had a normal rental car that handled it fine. about 5 years ago