The direct venting of septic tanks is very rare, but sometimes done.
If you are venting the inlet baffle side, you might be able to use this line. (I wouldn't, but you could be able to.)
If the vent is on the outlet baffle side, you are really risking sending solids into the drain field.
The thing you don't want to do is stir up floating scum and send it over to the outlet side. If you introduce sewage down into a vent line, you risk doing this. Sewage is supposed to enter the tank through the inlet pipe. This inlet is baffled to direct the flow down below the scum layer. If you look into a septic tank, it will have a thick layer of nasty floating scum. This is made up of grease and other floating stuff. You don't want that in your leach field. Clog that up and you are in for costly repairs.
The inlet baffle also directs flow down to help settle solids out. Scum floats and is trapped behind baffles. Solids sink and are trapped behind baffles. Mostly clear effluent passes through to leach field and is absorbed into soils. A good system will have an effluent filter prior to the leach field that gets cleaned out regularly.
You should instal a clean out between house and tanks and dump the RV there. A modest amount of RV sewage won't hurt a decent sized septic system if introduced properly.
Try to track down a set of plans for your system. Or, hire a pumper truck and try to see how your tank is baffled while he is pumping it down. You may be able to see how that vent line enters. A really good-been in business a long time septic hauler might even know your system, who installed it and how they preferred to set up baffles and vents. It the house isn't that old, you may be able to track down the company that installed it.
