Might be good to put in a low expanding foam insulation. I stuffed and cut Styrofoam and did all the patch work from the bottom the top lucked original except for a fine line hardly visible.
1950 Ford F1 street rod
1968 Baha Bug with 2.2 ecotec motor 170 hp, kingcoil
2000 National Sea Breeze 5th wheel trailer
1998.5 Dodge 2500 4x4 Cummins,4.10 gears,turbo,trans,injectors,oil cooler,lockers,edge EZ, 35" BFG's, air dog lift pump etc.
Funny you should post about your shower pan needing support, as I have just come in from working on mine! Due to a rotten floor under a leaking connection behind the shower stall wall, I'm now well acquainted with my RV shower stall! I believe the factory's intention and the poorly trained production work were miles apart on how a shower stall should be manufactured. The 3 braces that were supposed to act as supports under the floor were so poorly attached and the fiberglass so badly applied I'm in the process of rebuilding mine. I've removed the three braces (which were wafer wood) and replaced them with hardwood. I'll have to create a new floor bracing system, since mine was both rotten and inadequate for the job.
If in your case the floor is softor the bracing is insufficient in area, you will need to add more support before addressing the crack. Fiberglass is great stuff, but it has to be supported, it will continue to crack/get worse until it is stablilized. Once it has adequate support then you can expect a repair to last. If you just repair the crack without fixing the cause of the flexing, it will just happen again.
It turns out the crack in the showerpan was not the culprit. One of the water connections in the back of the "shower faucet" was cracked. Quick 3 trips to Lowes and I finally bought the right combination of elbows and connectors to redo the entire line.