We love Mexico and its people and have traveled there by air extensively over the past 30 years, particularly to the west coast. I would not take our RV there for a number of reasons, but that is just a personal choice.
However, the risk of violence has increased over the past several years and we have not traveled there since 2009 due to specific drug related killings, kidnappings, in areas along the west coast where we usually go.
As pointed out by others, it depends on where you go although there are increasingly fewer relatively safe places. The crooks rule in many places now and more often the crooks wear uniforms.
I found a website you might want to check concerning drug related violence in specific places.
Mexico drugs war murders data mapped | News | guardian.co.uk
Good luck in your travels. We hope to see you in Mexico again some day, but I am afraid it won't be soon.
2009 Newmar 42 ft. Allstar 4188, Wheelchair Accessible, 400HP Cummins ISL, Allison 3000, Mastertow Dolly, '98 Riviera