What are the long term chances of Robb keeping his kingdom of the North and River lands together long term considering the geographical and cultural differences? Assuming he won the war. What could be done to make the riverlands less of a cake walk to invade?

Long-term?  House Durrandon held the Riverlands for three hundred years, and House Hoare held it for three generations, and the Stormlands and Iron Islands aren’t quite as strong numerically as the North. 

Ultimately, it depends on what happens to the rest of Westeros. If Robb had fractured the Seven Kingdoms, and build a federation of the North, the Riverlands, the Vale, and the Iron Islands, and if the rest of the southron kingdoms were divided rather than united, it would be easy. The less Robb has to work with and the more the central authority has, the harder it gets.

In terms of how to hold it defensively, I think the key thing is to avoid trying to hold the whole of it. I differ from a lot of other writers in that I think the Trident itself is pretty defensible, as long as you hold Riverrun, the Twins, and a strong position at the Ruby Ford which may or may not be Harrenhal, and build a strong riverrine navy to maximize defensive multipliers and interior lines. The real problem is trying to hold territory south of the Red Fork, which lacks natural defenses.