The big difference between Mexico & the US is that the majority of violence in Mexico is targeted. Drug Cartels killing other cartels, the police, etc. You seldom hear of incidents like we see in the US almost daily these days, from school massacres to freeway shooters, etc. Certainly violence in Mexico is not targeted at tourists. Every time a tourist gets killed down there, you can almost bet they were involved in the drug scene and not ordinary tourists.
IMO opinion cartels keep a hands off policy on tourists. Tourism is Mexico's biggest industry and if they really want to bring the government down hard on themselves, the way to do that would be to target tourism, so they don't.
Statistically, an American has far more chance of being murdered in his own country than in Mexico. Now of course one has to consider that a lot of murders in the US are tied to domestic violence which is not that common on vacation, however it does put things into perspective.
I do not think any RVer has ever been murdered in Mexico. Plenty have in Canada & the US. I can think of 2 incidents in the US in the last couple of years and one in Canada for starters.