If it was mine, I'd get some very durable adhesive/sealant behind the shower panels, brace till cured, and then seal any exterior joints. One issue you may have is the condition of the back of the panels. If dirt, dust, or soapy water has gone on that surface, it needs to be cleaned for any adhesive to bond correctly. Fortunately, some adhesives will work in that situation. Silicone would be lower on my sealing list although it is an excellent sealant when in compression.
3M makes some excellent adhesive sealants as does Geocel Inc.