You could dump RV waste into a septic tank but it would be better to just dump the RV waste into a RV dump station or a public sewer system that you or a friend has.
If you dump the RV waste into the septic tank it will just fill your septic tank up faster which means you'll need to have the septic tank pumped out sooner.
It's not that expensive to dump the RV waste into an RV waste station which is a much better option but dumping the RV waste into a septic tank is okay if you want too.