Hi there, we also have a 2015 282 rk we picked up April 1 and love it too. We've been in it about 5 weeks now.
Yes, tere are 2 grey tanks, one for the kitchen sink and one for the bathroom sink and shower. The valve for the kitchen sink is under the slike near the rear of the unit. The bathroom is indeed connected with the black and grey. Just make sure you only open one of the valves at a time otherwise you may backflush some from one tank into the other. The rear gray tank has it's own connection and valve.
Mark, Vicki, & Scout THE dog
2015 Hemisphere 282RK
2010 F250 Super Duty
1992 Goldwing Aspencade