I've not had the problem you describe on my Jayco fifth wheel. However, the frame is reinforced with angle iron which runs from side to side at the spring hanger plates. I bought my trailer new, and that's how it came from the factory.
I think you are right that if your plates are bending, then some reinforcement is needed. I think that this sort of reinforcement is commonly needed due to the forces trailers encounter when making turns.
I wish I could post a picture of my trailer's suspension, but my trailer is not accessible to me at the moment.
I hope you will get more replies with suggestions.
There's lots of advice and information in forums... sometimes it is correct. For example, all of my posts are made by a political appointee who got the job as a reward for contributions to my diesel bill.
2011 Jayco 28.5RLS; 2008 Chevy Duramax; Pullrite Superglide Hitch