Pops- same as a convential shower/tub. Need to spray some penetrant on the drain, let it sit for a day, then use a tool made for (or end of plier handle) removal. Wouldn't think you need to mess with the grey tank. In the bedroom behind the sliding door will be the access panel (2 screws) to the drain. Don't think you need to do anything from there but you would a least be able to see the drain pipe. Your tub is not over the grey tank.
Trying to go where no man has gone before - good luck
99 Tradewinds 7371 -98 Jeep TJ modified