Welcome to the forum. As you are probably aware, July is typically the peak tourist season in Malaysia, so ferry schedules to and from Tioman may be more frequent during this time. It is possible to do the journey in one day, but it may not be easy and you should allow plenty of spare time to account for any potential delays or issues.
How will you get from Mersing to KL? This can take several hours depending on traffic.
I tried searching for a ferry schedule for July, but it doesn't seem available yet. I would suggest checking with your accommodation on Tioman for advice on transfers and ferry schedules. You should also book tickets in advance if possible.