The Kingdom of Rhovanion was a kingdom on the eastern side of Mirkwood and between the river running. It stretched from eastern Mirkwood, down to Dorwinion. Vidugavia became the self proclaimed king of this region after becoming the most powerful "Northman Prince."
The Kingdom of Rhovanion saw many battles with the Easterlings, it was Vidugavia's alliance to Gondor that bought him a substantial victory against the Easterlings of Rhun--but it also brought the hatred of the Easterlings. For many years to come, the Easterling tribes of Balcoth and Wainriders would harass and take over the region. It was not until the coming of Eorl and Cirion that ultimately defeated the Easterlings and ended the war.
It should be noted that many wikis state that the Kingdom of Rhovanion was utterly destroyed--the justification for this is that there is no evidence that it still exists--but Dale and Dorwinion seem to be off shoots of this kingdom, and these two areas may be justification that it does still exist, and there is a line on pg. 172 of the Hobbit that reads: "The wine, and other goods, were brought from far away, from their kinsfolk in the South, or from the VINYARDS OF MEN in distant lands."
It is perplexing to think about, if Dorwinion was originally part of the Kingdom of Rhovanion, then how did it survive? In the Balchoth Wars, the Easterlings enslaved many Northmen and probably used Dorwinion as a feeding ground for their troops, then there is the line about Bladorthin to consider as well. But the Balchoth actually set up in the region that was the Kingdoms of Rhovanion. Doesn't that mean they were destroyed? Probably, though it may have been re-built by Bladorthin or his father--most likely his father.
While much of the Kingdom of Rhovianion must remain in the realm of speculation, we know that Vidugavia had more than one child and that Rohan could claim lineage to him--but Marhari is never called a king, but is descended from one of Vidugavia's children. This is still plausible if there were other sons that Vidugavia had and their descendants hold claim to the throne at the time, which would mean that the ancestor of Marhari was a Northman Prince from this kingdom and would thus fit the canon of Tolkien's work for The Hobbit, Dorwinion, Bladorthin, and Rohan.