Book was most likely a paperback book from the 1970s or 1980s.
Possibly set in a post-apocalyptic (possibly plague or pandemic) United Kingdom (maybe Scotland) or Ireland?
A American(?) Soldier arrives in a small town driving a (Sherman? or M1?) Tank. (Maybe the sole survivor of a nearby Army Base) He fends of a raiding party trying to rob the town (either a roving band or a party from a nearby town).
The Towns people celebrate. Shortly after it is discovered that some of the women he has entertained are pregnant. (Every other man in town is sterile.)
So they set him up as the Lord of the town with "First Night" rights to the brides of any couples who want children. Called "Droit du seigneur".
I am pretty sure the Tank was nicknamed "The Fist"
As I recall the book seemed to be almost a collection of stories as opposed to a smooth flowing continuous story. But all concerning this town, the Man and his tank.
The book covers an extended period of time, as I seem to recall the last story/chapter he was concerned with what to do about "First Night" rights with his "Daughters" who were now becoming of age.