I am running a mega dungeon, and the party has been completely finishing off the parts of the adventure that are more than 5 levels below their party level. This means they are finding plot points and stuff that they would have otherwise missed. But their experience points are tending to zero as it is only the minor parts that are even close to their level that they get XP for. I don't want to escalate the danger as this will only mean that from now on I have to make it harder and harder. Should I just give them a flat roleplaying award each week, after all they are finding and resolving all the plots even if it gives them almost no XP? If so what seems like a fair amount?
This problem will solve itself
First, as long as they enjoy playing like this, with no real danger or risk of death, and no real challenge, but full story exploration, then it is no problem to have not a lot of XP. I have played side adventures for my groups that were much lower challenge ratings than their PCs, and the players actually tend to enjoy being able to flatten their opposition for much longer than a DM might expect.
Of course, character growth is one of the carrots that make RPGs fun, so it may well be that the players feel they should at least gain some levels every now and then or see progress towards it, or their enjoyment of the game might suffer. This is probably more a function of time spent playing the character, than time in-game, because it is the players that want to experience some new power. In my experience, if you throttle XP progress too close to zero, and players feel they make no meaningful progress, they become unhappy (I did this with a group which I did not want to progress too fast and overshoot the challenge range of adventures I had planned).
On the other hand, if you start giving out meaningful amounts of XP to them that will let them gain levels, then you will only make the problem linger and continue for longer.
Give them just enough milestone based XP for clearing story points (or maybe as you suggest XP for role-playing if they do that well) so they can see light at the end of the tunnel and remain engaged, but not so much that the situation gets extended too long:
If the PCs are five levels higher than the opposition, then instead of granting enough XP to level up with each dungeon level cleared as is commonly done with megadungons, you could give them enough to go up every 3 levels. This way, they will catch up with the dungeon not much later, but they can see some meaningful if small progress while doing so.
With their powers, they should be able to clear these lower levels fast and easily, and eventually will get into those where life gets a bit more dangerous, and the XP rewards will increase naturally. Once this happens you can scale back the story/role play awards. Or, if you leave them at the same absolute sizes, they will quickly become relatively less important compared to combat XP by themselves, due to the nonlinearily increasing XP needed for every level.
May I propose non XP awards as one possible solution? This can also be clearly explained to the players. Power, Prestige, Wealth, and Property are things that drive many people in real life and from which they (and humans in general) tend not to learn much.
However, one needs to be just as careful and measured in giving out those rewards, lest an influencing PC raise an army of followers or become owner of a chunk of the dungeon (and then have to manage the endless complaints and requests) ... which might be a fun side quest in its own right. ;)