Read it around 1978-9. Wizard plucks an M-113, about to be RPG-d from the jungles of Vietnam, with its crew and dumps it into a medieval hack and slash land with various non-human races and some magic.
The new king there is near-sighted and wasn't anyone's first pick when his dad died, but he ends up doing a good job. I believe the APC meshes in with the whole war of succession bit.
The US soldiers have a number of adventures, eventually barely kill a dragon (wizard had aimed for a tank, not an APC) and get sent back home, 10 seconds earlier, where they expect the unlucky RPG gunner and nuke his butt.
There was a second book as well, tank commander goes back to live in sword and sorcery land, but I don't think the author did much else after.