In many jurisdictions— Including Louisiana, where I'm from— the water from a private septic system can only be used to irrigate pasture and crops that are NOT intended for human consumption, and only if the system is inspected and approved in writing by the appropriate state Office of Public Heath. The treated solid waste (actually a slurry) has similar limitations as fertilizer.
RV tanks don't treat waste, they only store it. Untreated human waste— including the effluent from an RV blackwater tank— can only legally be dumped into a state-approved treatment system. Graywater is more of a gray area, but I'd consider it the same as blackwater; any witnesses, inspectors, etc. are not likely to know or care that you've got two different holding tanks, only that you're dumping where you're not supposed to.
Direct discharge of untreated waste from your holding tanks onto the ground is illegal in most jurisdictions. The Feds could even get involved if someone determines that you're in violation of the Clean Water Act. However, TG Twinkie, by dumping his/her holding tanks into a private septic system, is probably legal, because the effluent is treated in the septic system before being discharged onto the fields.
Weigh your civic conscience and the risks, and decide for yourselves. But please, don't dump your tanks on the ground if I'm parked nearby.
Engineering: Finding complex solutions to simple problems you didn't even know you had.