I think it's generally more enjoyable (for the RP'er) to reward good roleplay with in-universe roleplay awards, rather than XP.
The good RP'er has a better in-game reputation amongst the NPCs (either friendly or feared depending on what they go for), forms alliances or friends that will pay off later, may get a better reward because the NPC's are more impressed with them, etc.
The combat monster gets their enjoyment from curb-stomping monsters, or pulling out a win when the odds are against them. They'll barely remember RP rewards, much less care about them.
(This is an extreme example, as there are players who care about both RP and combat effectiveness.)
I wholly agree with this; I consider myself a powergamer/optimizer (I will play any kind of character, pregen, whatever, but I definitely have a preference), but I definitely appreciate in-game rewards. Honestly, I really could care less about getting XP at this point, I just enjoy playing the game, but if my RP were to earn me a serfdom, or a business with some employees, or a collection of slaves, some things that I could make decisions about that had long term ramifications in the game world, I would put more effort into my RP. By the time you hit the mid levels it's very possible to have more gold than you'll spend in your career, and you're so busy as an adventurer there's little point to spending it on the construction of a new keep or whatever. But being gifted with already established properties is actually kind of awesome.
In one game we killed a hoard of vampires who had taken up in an old estate (big stone mansion, not really fortified) and we spent a good three sessions ignoring the quests at hand and just working on our new guild-hall (had a well of worlds, part of the quest line, trying to be brief). My character was a slave to an efreeti from the elemental plane of fire in the city of brass, and there were a bunch of dragonborn slaves that I was raised with, so we spent a considerable sum of gold and did some quests for the efreeti to get their freedom, so then we took all the freed slaves to our guild hall and helped them build up a town with farms and mines and my paladin established a temple (recently had adopted Kord) and the whole group was WAY into setting up their own little aspects of the newly formed guild (we were around level 8 when we got the property and had been amassing wealth for levels with little opportunity to spend money, the monk set up a little dojo, the druid locked himself up with the well of worlds, the rogue hired a halfling engineer to build him contraptions (including the paladin mounted halfling launcher which was basically a modified crossbow that let me shoot the halfling rogue at dragons because neither of us had ranged attacks), and all sorts of stuff, rescued some town folk from an undead army and they moved in, good times).
If you want to reward RP strongly, and you've run out of cool mundane stuff like titles, lands, retainers, etc., consider minor boons (rewards from a devout cleric's deity, or the spirits of the wild help the dedicated druid, etc.) or the non-combat oriented wondrous items. An RPer is going to be ecstatic if NPCs start handing out single use items that do random crap to their friends, and, you can always trick the munchkins into thinking they aren't being hosed by giving them magical equipment for their minor attacks (your barbarian already has a +2 greataxe? toss him a couple returning handaxes, no plus, they just count as magic and return) or tricks. "Yeah, the manifestation of nature bestows a boon upon you too, your save DC for the spell minor illusion is +2" or something similar. Just a tiny mechanical buff to them that doesn't really change the outcome of their broken tricks all that much, but they still go "Hey, that's a reward I care about" even though its effects are actually negligible.
It might be possible to make a fun game where TPK happens regularly and unavoidably but it wouldn't be easy.
