According to Subaru, you can only flat tow a Crosstrek if it is a 2021 or later model year and does not have a Lineartronic continuously variable transmission.
If your car doesn’t fall into this category, your best bet is to use a trailer to pull your car behind the RV. Flat towing a car that isn’t meant to be flat towed can cause serious damage to the transmission or powertrain and result in very expensive repairs.