Generally the event you will be at is going to be at a fairground.. And they will have at least one dump.
Your city (if you live in a city) may have a dump. Mine did but nobody knows where to find it (No comment, typical government operation, I should know, I'm a retired government employee)
As someone said, Several Gas stations, some charge a fee.
The web page linked to is a good one but the advice is call ahead to see if someone knows where to find the thing.
Home is where I park it!