The 'best' possible use for Leonardo's tank would be as a support battery if, brought once in position (with the help of oxen or horses) it would be able to rotate like a merry-go-round. Thus while one of the guns was firing the others would be in the process of progressively reloading, allowing a (relatively) high rate of constant fire against a given target. But would it be more efficient than the same number deployed side by side ?
Note that Leonardo's tank requires breech loaders, but such were known from the beginnings of Western artillery and Lazare Carnot in a review of French ordinance in 1812 mentions that the swivel-mounted very light guns used by French navy were breech loaders.
The Hussite war wagons were purely defensive, basically elements of a mobile fortification (like the later
Gulyay-gorod), since they were 'fighting' from the side and had to form a laager. As an offensive device there are medieval illustrations of very wide but short wagons *pushed* by horses or oxen with spears in front and scythes on the side, carrying handgunners or an organ gun & billmen but I doubt such were ever actually built. A
more promising variant could be dreamed of later, say by the mid-18th C. (supposing an equivalent of the Watts steam engine was developed earlier than OTL