The best solution would be for a plumber to install a "clean-out" that connects to the baffled inlet pipe. That way, you know that you are dumping in the right place. While you are at it, maybe consider having the plumber add an anti-backflow device between the pipe and the house, so a septic tank backup backs up at the tank, and not into your house. Caveat: The backflow device will need maintained periodically or else it won't work when you need it the most.
Actually, the absolute best solution I have seen was a separate septic tank and two leach fields (with a large valve to swap between them yearly) just for the RV. However, that might be a bit of overkill.
