RunSlashCmd isn't a function provided in the wow api. It's possible to make said function but it's just adding in an unnecessary middle-man.
All slash command handlers are stored in the SlashCmdList table. You just need to find the hash index of the command you want and run it as a function. i.e. if I wanted to call /stopwatch 30
Code:
SlashCmdList['STOPWATCH'](30) -- 30 second timer