Legend
X - 1.5" FIP adapter
T - 1.5" barb to MIP
O - 1.5" slip joint
œ - sink drain pipe to 1.5" pvc adapter
§ - 1.5" splicer
x 0.5" pex


SINK

|                  |
|                  |
|__________________|
        T <- 1.5" strainer bottom
        X 
        |   
         \ 
          | <- 1.5" flexible tailpiece
           \
            O
            X
            |   ___XT <--- switch from drain 1.5" to 1.5" vinyl pipe
  P trap--->|   |   |
            |___|    \
                      |
                      |
                      \
                       \ <----------- 1.5" vinyl tubing from p trap to floor of van
                        \
                         |
                         |
  VAN FLOOR              |
                         T
_________________________X______________________________
_________________________||_____________________________ <--------- small piece of 1.5" PVC pipe with two
_________________________X______________________________            sealed into floor. 
                         T                                          1.5" fittings on both sides.
                          \
                           \
                            \
                            / <--------- 1.5" vinyl tubing under car
                           /
                          /
                          |
                          §
                          |             (goes under car, zip tied to frame)
                          \
                           \
                            \
                             \
                              \
                               \
                                |
                                |
GREY WATER TANK                 | <------------------ Vinyl pipe to 1.5" barb, directly to grey tank
________________________________T__ 
|                                  |x <-- vent
|                                  |
|                                  |x---[] <- electronic release valve
|                                  |
|__________________________________|

• Form-n-fit tailpiece to p trap
• P trap to 1.5” vinyl tube -
• vinyl tube to 1.5” female threaded trap adapter on floor
• 1.5” trap adapter to small piece of 1.5” PVC pipe
• 1.5” PVC pipe to another 1.5” threaded trap adapter
• 1.5” trap adapter to 0.75” vinyl tube
• 0.75” vinyl tube across bottom of car, to 1.5” female threaded trap adapter on grey tank

Alternatively: no vinyl tube inside car, only PVC pipe down to the floor

Still need to find out:

• P trap to 1.5” vinyl tubing — how??? how to connect barb to sink drain?