As noted, John Cena returns to WWE television on the July 4th episode of SmackDown and WWE has been teasing that he is a "free agent", which would indicate that he would be able to appear on both RAW and SmackDown.
It's not known if the "free agent" storyline is being done to allow Cena to appear at RAW events when needed, however Cena is not currently scheduled for any WWE Monday Night RAW tapings through SummerSlam, including the post-SummerSlam episode of RAW. We have learned that Cena is scheduled for every Monday SmackDown house show that he is available through SummerSlam, including the post-SummerSlam SmackDown live event in Albany, NY, so he would not be able to work RAW on those Mondays unless it was close by.
Cena is also not currently scheduled for any other RAW live events through SummerSlam other than the two we mentioned, and is scheduled to work all the SmackDown live events that he is available during that period. Cena is also scheduled for every SmackDown Live TV taping through September with the exception of the August 8th episode.
Cena will also be appearing at the next SmackDown pay-per-view, WWE Battleground, on Sunday, July 23rd, which is the final SmackDown pay-per-view before SummerSlam. He will not be appearing at the last RAW pay-per-view before SummerSlam, Great Balls of Fire.
While WWE is running with the storyline that Cena is a free agent, he is unlikely to appear on Monday Night RAW anytime soon.