Not sure about adding a grey tank which was not included on this model year. Also, shouldn't your black tank have an area for it under the trailer. As for the shower and toilet pan. The toilet you have is in the place that the shower was originally - it should be sink in the middle - toilet to the left - shower to the right. See this site and scroll down to the bottom for photos of various years including yours.
There are some place that sell salvage tubs - try searching the internet. Your best bet, if your game, might be to trace the the floor space and have one built at a local machine shop out of aluminum. Another option is to do it like a home shower and build a front lip - line it with shower pan liner under light weight tile backer board and build it out.