Just let you all know that this seem to be a problem on a lot of heavy 5er trailers. I had a 2008 Keystone Challenger, after the warranty period run out, I noticed the flexing, about a 1/4 inch. Keystone washed their hands, sorry out of warranty, I was very disappointed with Keystone not standing behind their product. The Dealer told me that a lot of 5ers develop this problem, but it should not be a safety issue.
I was not happy with the flex and traded the unit against a 2015 Sanibel 3250 and hopefully I won't run into the same problems down the road when I have more miles on the unit.
Should it happen I hope Prime time stands behind their product.