Welcome to the forum
The mold lines are only on the exterior surface, and as long as you maintain your seams/seals there shouldn't be any issues with what's under the membrane. It's not just Keystone trailers that have this happen.
2012 Copper Canyon 273FWRET being towed by a 1994 Ford F350 CC,LB,Dually diesel.
Airlift 5000 bags, Prodigy brake control, 5 gauges on the pillar.Used to tow a '97 Jayco 323RKS.
Now an RVIA registered tech. Retired from Law enforcement in 2008 after 25+ yrs.