Not Medieval, Dark Ages
If you think about it, the way all those nobles got their land and titles is that their great, great, great, ... great granddaddies were very successful military men who 9back in the badly misnamed "dark ages") took by force and controlled a piece of land or were lieutenants of very successful military men who were entrusted with the control of land that their boss took. Of course, generations down the family tree, these aristocrats probably don't resemble the warlords they were descended from in terms of ability or temperament, but that's the way most of their families came to power.
Now think how this would work in DnD...
In a frontier wilderness region, powerful characters clear the area of monsters and bandits. These characters are adventurers, mercenaries, leaders of popular uprisings, whatever.
Eventually one or more of these characters takes the Leadership feat, sets up a stronghold and provides protection to the whole area. Other characters do the same. Maybe these are cohorts or allies of the first character and swear fealty to her, maybe they compete with her and there is a series of small wars and someone ends up on top, whatever. Eventually you have a hierarchy of warlords that looks like a feudal kingdom. As this settles down and the region becomes more peaceful, this consolidates into a hereditary feudal kingdom with a hierarchy of nobles descended from the successful warlords.
Of course as the region becomes more and more peaceful the key skills for the nobles become less and less the spellslinging and sword swinging skills that cut a kingdom out of the wilderness way back when, and more and more the diplomacy, bluff and sense motive skills and the alliance and dealmaking that characterise the politician or, in DnD class terms, the Aristocrat NPC class.
Modern adventurers are thus both a threat and a possible ally to the current nobility. The represent the nobility's past. As such, history teaches how powerful these adventurers can be at changing the social order - that's how the nobles’ families originally got where they are. But these adventurers *are* very good at taming wilderness areas on the kingdom's borders and thus expanding the realm. They also provided access to the families lost ancestral skills where the peace of the kingdom is threatened.
So what would be the attitude of nobles to adventurers? A combination of suspicion and manipulation which could swing into hostility or and alliance depending on the adventurers attitudes and behaviour.
Just my two coppers worth...