This depends on what exactly you want to control and where.
All showers need valves. They can be for the water supply, balancing the water and control of the shower. Without the valves, your shower will leak, causing damage to the surrounding areas.
Showers in mobile homes are no different from showers in a house. Most shower problems lie in the control section and many of them involve sediment or other particles blocking orifices. Taking a control valve apart to look for this is not difficult, but remember to lay parts in order of taking apart. You may need some new parts, if so take the old parts with you to the store for comparison.
You need to phrase your question better.
I like VFD control. A VFD varies the speed of the motor which therfore varies flow. A control valve simply restricts flow. Control valves usually need intrument air compressors and have lots of moving parts and things that need repair. VFD control has no moving parts and uses less energy when running at a reduced capacity.
First decide if you want to take a shower valve repair yourself, or contact a plumber. If you decide to repair the shower valve yourself, first you will need to shut off all water to the shower and tub.Next, turn the valves to the on position to relieve any pressure, and remove the set screws. Next, remove the trim ring and save all of the gaskets. Take all of the valve parts to a home improvement store where they will assist in finding you all the replacement parts. Lastly, you'll need to put everything back together in reverse order.
You will just need new washers.
I believe there is a very small hole at the base of the handle knob, on the side, which is a bit hard to see. You will need a small Allen wrench to engage it and turn counterclockwise to loosen. These valves are always leaking and freezing up. Consider a single control with a cartridge that is easy to replace.
Yes, you should install a vent fan, to help keep mold, etc. under control.
No, since there are no valves, there was no need for a cam shaft or belt. Or, all the parts that go with it. No header mechanism required.
You may have a broken pipe or have not paid your water bill and they turned it off at the main. Check the main for a tag from the department of water and power.
Since you will be draining some of the water first there is no pressure to worry about. Shut off the water supply (for a shower, it usually means you need to shut off the water at the meter) and open the shower valves (hot & cold, if applicable) then find a sink, toilet or outdoor water faucet at some level below the height of the items you are to replace. You don't have to open them for long... a few seconds will do. This allows air to enter the shower valves and pipes and will drain to a point so that dripping water will not be a problem.
To fix a low pressure issue in your shower head, you can try cleaning the shower head to remove any mineral deposits or debris that may be blocking the water flow. You can also check the water supply valves to ensure they are fully open. If these steps do not improve the pressure, you may need to replace the shower head or consult a plumber for further assistance.