Help with Sewage pump and basin and how to tie in with gravity system

I have a existing septic system and need to get grey and black water from an RV 100 feet away where there is only a 6" drop. I have purchased a Liberty P383LE51 pump and basin system with the 3" discharge (that is the same as an existing line from the old house being torn down).

I have an apartment (new construction) exactly half way between the septic and the RV that could go gravity fed as the slope could work with several feet of drop from the 50 ft distance. (the apartment is being built with floors at 3' above the ground)

My question is how to make the apartment and RV work together. Should I...
1. Run 3" from the RV site below the apartment site and set the basin a few feet below grade so both drains go to the basin inlet and pump out on a single 3" to the septic, or
2. pump from the RV spot all the way to the septic and gravity feed to a tee at the septic entrance?

Or is there a 3rd alternative in here somewhere that is the best way?

RV -- 50' -- apartment --50' -- Septic tank
little to no rise/fall over the 100'

Thanks in advance,
Rob