Sorry, but I literally mean I fill from the tap full blast into the tank. Im D.I only
Then an ordinary toilet stop valve will be fine.
However, there are a few things you have to consider. Because the valve operates on a floating ball that operates the valve, you are never going to be able to brim the tank so you will lose some of your tank capacity.
The other consideration is where you are going to drill your hole in the side of the tank. If it is too low then you will lose even more capacity. If you drill it too close to the top of the tank the valve won't switch off. Also bear in mind that the valve 'wears' in time and won't switch off at the same level over a period of time as a toilet cistern does - overflow.
You may find it best to fit the valve in such a way that the float can use the space in the filler cap and find a valve that has a small float. Depending on your water pressure you may have to find a stop valve that can be modified for high and low water pressures.
Your stop valve may not last as long as it would in the toilet as it will be subject to the water sloshing around in the tank when the vehicle is moving.