No, treasure is not low in Rappan Athuk
But that is due to the adventure's nature: It is a sandbox campaign.
What does that mean? There are treasures hidden in places you can't even imagine, but you have to look out for it. Not all treasure is in plain sight, rewarded after defeating enemies. Especially because you will hardly defeat all enemies in the campaign.
Sandbox campaigns in general have an issue where random encounters will not generate enough treasure, while at the same time inflating the XP (and thus the group's level), which makes it so you need even more treasure to be within the expected for your level. This may also happen if the adventure has a lot of traps (and it does), since those naturally won't have a lot of treasure (dead enemies are probably looted by denizens).
As a result, you will feel like the adventure is not rewarding you enough, and that even may be true for the first few levels, or based on the direction the adventure has moved. If your group happens to have picked the leads that won't award very well, it's just bad luck. If your group had a lot of outdoors encounters, either while travelling or while camping and got attacked by random enemies, then you likely won't have a lot of treasure either.
But keep in mind that it's ultimatelly a dungeon campaign, meaning that the focus of the designers was in the designing of the dungeons, and the deeper you go, more treasure you will be rewarded.
I decided to be a support-ish character and invested in the Create Wondrous Item feat so we can have more suitable equipment to stay independent from any merchants. It also saves half of the gold that we would have to pay, but the way things are now, it does not compensate for the shorting in our wealth.
Crafting magic items does not shorten your wealth. It transfers your wealth from "unwanted treasures" to "wanted treasures". Example, if you dislike that an enemy dropped a +1 halberd because nobody in the group can use or wants to use it, then you can sell it for half price (market value 2,000 gp, thus 1,000 gp) and buy material components to make that +1 longsword that your fighter really could use (1,000 gp in components). See how the net wealth did not change? It is merely transferred from one type of item to another.
Have a little chat with your GM
See if he isn't ignoring the fact that some random encounters should have treasures with them, and he happened to forget about them because such items aren't always listed in enemies's statblocks. This doesn't have to be monetary or magic items, but are supposed to increase the party's wealth accordingly.
For instance, you know that Bonesuckers (and Vrocks) have a Standard treasure, right? Even if the monster can't really equip magic weapons and armors, their lair should have the remains of their previous victims, or you could rip their belly off and find that they had eaten a few precious gemstones, or even, you heard that there is a merchant paying a good sum of money for a few bonesucker tentacles.
So, it's possible that your GM forgot to account for this extra treasure from those encounters, and that's why your group is under the average wealth for your level.