First post, First fifth wheel, First question!

No. You do not need nor do you want to place a brace under the slide out. If for some reason thw trailer droops a bit you could damage the alide out.
With a fiver this age, would suggest that about the first thing you do is get up on the roof and give it amcomplete inspection. Look at the condition of the rubber membrane and all the seams. Be very careful about anything you use to seal the seams. Sicor lap seal is about the best for roofs.