Hi @kingcoolance,
There’s lots of missing information here that’s required to answer your question.
In particular:
- Which Raspberry Pi model?
- What is the output capacity of your 5V power supply?
- How much current does each camera use (this can depend on the camera model, and the streaming settings you configure)
- How much current do other attached USB devices use (the fan, and any others)?
To avoid an “XY problem” scenario, what do you need these cameras for? Are you streaming video to help with piloting, recording video for data purposes, both…? If you’re only recording you can potentially have independently powered cameras with onboard recording capability (like an action camera).
In terms of powering it may help to power the cameras off a separately powered USB hub (which avoids pulling excessive power from the Raspberry Pi), but that’s also dependent on other parts of your setup.
20A at 13V is not a very large power capacity. How many thrusters do you have? Do you need to use them all at once? When powered at 13V, how much current does your thruster model use at different PWM levels? If you have a well-defined relationship in the thruster’s technical information (e.g. from the manufacturer) then you should be able to calculate the maximum PWM, at least as a starting point that you can later tweak 
Is your vehicle running ArduSub firmware? Do you have a joystick connected? If so you can map it as an Actuator and use joystick button function inputs to control it. If you want on-screen buttons or custom functions then that’s also possible.
That said, I’d recommend using Cockpit instead of QGroundControl - it’s more configurable, supports viewing multiple video streams at once, has more powerful customisation options, and we can support it better. We’ve been developing it as a QGC alternative for years, and are in the process of migrating our guides to recommend it for all ROV users, with boat users soon to follow…