I see a lot of issues with roofs sagging. With an A/C on the roof the weight of it can cause the roof to sag if it's not supported. Interior braces should be added to help prevent this.
Of course another issue can be water damage where the A/C gasket leaks. This can happen after several years as the gasket dries out or the bolts loosen. Water that pools near the A/C will cause leaks, and that is the first sign the roof is beginning to sag.
Our roof support bars. They attach to the sidewalls.
Interior sidewall attachment
Of course every pop up model has a different roof line. Some have a slight bow while some are flat. Finding roof braces for your model may be like looking for the needle in the haystack.
A lot of people have used Unistrut to fabricate braces. Take a look at this YouTube video to see one guy's method of using Unistrut to build a roof brace. He did a great job.