My last TT (a Keystone Passport) was like that from day 1. The exterior side walls were all very solid, but the back exterior wall, when pressing on it from outside the TT, would "wiggle" for lack of a better word. There is no difference in feel between the side wall versus rear wall versus front walls of our current Flagstaff. All of the exterior walls feel very solid.
Brad, Sandy, Amber (shephard), Niko (shiba inu RIP little buddy)
'12 F-150 XLT SuperCrew 4x4 Ecoboost with MaxTow
'12 Flagstaff V-Lite 30WRKSS