One first has to define "accident". e.g. fender bender? Someone else hitting the trailer/RV? The trailer/RV driver rear-ending someone, or turning left in front of someone, or another type of accident that really doesn't have anything to do with being in an RV or towing a trailer? Or are the only accidents worth considering those that cause bodily injury, or roll the trailer/RV (much harder for a MH)?
I don't believe there are good statistics available for anything other than accidents that result in fatalities:
http://www.nhtsa.gov/NCSAhttp://www.iihs.org/iihs/topics/t/general-statistics/fatalityfacts/state-by-state-overviewAnd the number involving trailers is so low that it doesn't reach statistical significance, and is not tracked as a separate category.
http://www.allcountries.org/uscensus/1037_fatal_motor_vehicle_accidents_national_summary.htmlMotorhomes fall under "Other Vehicles". Cars pulling trailers aren't tracked separately, and there are other things that also fall under "Other Vehicles" such as ATVs.
Personally I've never had a bake system malfunction in any of my trailers that wasn't caused by me forgetting to plug in the pigtail (which I've done twice in 15 years). I'd bet (but there are no statistics available to back it up) that far more accidents are a result of either people cutting others off (forgetting how long they are) then over-compensating, or caused by improper hookup - loading/balance/uncontrolled sway/poor hitch configuration and similar issues, than are a result of brake system failures, defects, or inadequacies.