We have made the trip to Chetumal across the border from Belize in an s.o.b, travel trailer.
Yes, you are safe but take the well known precautions particularly in the states in the news for cartel violence.
Not only will you need Mexican insurance but Belize insurance as well. You will also need a visa to get into Belize. Insurance and your visa can be obtained at the border
If you have not traveled to Mexico before in your rv, I suggest an easier trip first. Or a caravan, there are caravans going to the Copper Canyon and other popular places. I don't think there is a caravan going to the Belize or the Yucatan.
Bring money, Belize is beautiful but pricey. Cost of living in Mexico is similar to the U.S. If you want to get anywhere in Mexico, you will have to take a toll road and with four axles on the ground you are going to have to pay some serious tolls.
Speak some Spanish, as outside of the tourist areas few speak English and in fact understand my badly accented Spanish with some difficulty.
Most importantly, have a spirit of adventure, be flexible and try not to be nervous. Travel in your Airstream in the third world is very different than going from campground on the interstate.