For starters there is a master thread covering all things with regards to selling to Vroom.
Also unless I’m missing something it doesn’t make sense that your payoff now is lower than what your RV is in 3 months time. And AFAIK the state of CA has a sales tax exemption if you buy and subsequently sell your car within 10 days. I would call MBFS to verify exactly what your payoff is and do some more research on the CA tax exemption.