You really shouldn't need trim tabs on a RIB this size, but if all else fails they might help.
Where are the aux engine and extra fuel tank fitted? Are they right at the back of the boat? If so, relocating them forward would be the best option -- trim tabs are a poor second best to balancing the boat properly.
If that's not possible, then trim tabs are relatively easy to fit. You may find that a pair of simple fixed trim tabs would be good enough, which would save the expense and extra installation work involved with hydraulic tabs.
The drawback will be a reduction in efficiency as you will be using some of your power to dynamically balance the boat rather than push it forward.