Carriage introduced and made the full wall slide since the 2009 model year and made it through their demise. Lifestyle has continued its production. It is one of their most popular models and according to owners on the Carriage/Lifestyle forum, there does not seem to be any issues.
I ordered one back in November and should receive it in a few weeks. I have never heard of anyone having any problems with sagging. The Lifestyle frame is very strong and in speaking to the Lifestyle rep while at the Tamps RV Show, he was explaining how the slide out ceiling and frame is made and strengthened.
There are many motor homes with full wall slides so I would imagine they have figured it out. Good luck. Phillip