Shower drain retro install

[Borden]

Boler has a molded fiberglass floor with layer of ply and vynal flooring on top below molded two peice bathroom stall.
The recess in floor of washroom is shalow and 3 1/2" wide wish to know best place to look for a drain fitting? Also considering adding a SUBURBAN WATER HEATER -SW3P-GAS 3L unit for weight and size reviews on 6L unit look good?

[Borden]

PS the floor is not drilled yet but plumbing was cut in below prior to us getting the trailer

[Borden]

did reseach and will install drain this week, and take pictures of progress.