Shower Floor and Toilet riser.

Floor flexing there is very common, if you do a search you can find various fixes, most of which are pretty simple involving placement of some blocking underneath. If you have a crack for sure check for water damage underneath.

Might be pretty hard to reseal that crack, sealants that can take a lot of flexing generally aren’t real good as adhesives. I think I’d try stabilizing the shower pan with blocking as much as possible and then try some 3M 5200 marine sealant on it. I’d be curious what others here would recommend.

Just ventilation, the battery charger is under there to the right. Remove the screen and you can inspect and add blocking.. If you never plan to use the shower then you don’t really need to worry about sealing the crack, just reinforce it under there to stabilize it.

If you do plan to use the shower, the reinforcement under the floor, mentioned above, is necessary, but you will also need to fix the crack.  That should not be done by applying a caulk or sealant, as it does not stop the mechanical movement of the fracture.  You really need to do a fiberglass repair.  If you have no experience with this sort of thing, it's probably best to have a body shop that does fiberglass auto body repairs or a boat yard fix it.  If you can remove the shower pan, the fiberglass patch can be put on from underneath and the crack in the gelcoat touched up.  The hard thing is to match the color, if that is important to you.  

I think some of you missed the intent of my question. They have many new great epoxy proxy products. The crack is the least of the problem. The pan and drain flex. You can feel it when you step into the shower/toilet. How can I support the pan floor. The vent below the door is the only opening. How do I install anything thru that small opening. A contractor suggested expansive foam. I do not feel good about that. Help!!!!

Jato's fix is probably sufficient to fix the shower pan flexing.  Another option would be to use a piece of 1x2 horizontally mounted on top of the support block [the 2x6] to extend the support out a little further from the edges of the 2x6.  The space is awkward to get into, but it can be done, especially if you have a contortionist or a mechanically inclined child available to help.  If you still can't get in, you can cut the hole a little bigger and get a bigger grill cover to close it.