Update
I spent the day yesterday removing the other tank, removing the access plates on both tanks which was not easy. I flushed out both tanks really well and afterwards inspected with my camera just to make sure I got it all out. The right tank had considerably more flakes than the left. Interestingly, the right tank was also the one that looked like there *might* have been slosh in some specific areas that was brushed on and then later removed - but maybe not removed completely?? This is just purely a guess based on how it looked. I'll upload photos later.
I spent some time digging around and found the builder's phone number and had a nice chat with him earlier today. He sounded like a really nice guy who absolutely loves to build. He started two more projects after the RV4 (which he won a best home built award for), but never finished due to health problems. If feel bad for him. And as for the fuel tanks, he said he did not use slosh. I asked him again if he might have used it even in a few small areas and he said no, although the tone of his voice was not very convincing. (Or maybe I'm just reading too much into it)
My best theory for what happened is that the builder used a *small* amount of slosh and then later removed it, but not completely. And the reason why I think this is because of the way it looks - it looks like there was something on the walls of the tank and then wiped off - kinda like when you wipe your kitchen table and the stain has not been completely removed and you can see the trace of it. I took some pictures, but they didn't show well. I'll try to get better ones in a few days. Anyway, this is just a theory. I really enjoyed talking to him and felt he's a really nice guy - but maybe just didn't want to admit the he used slosh?? Again, I might be totally wrong.
I feel like this is more likely than that white stuff coming from the fuel supplier. I get my fuel from the truck at my home airport (CPM) 90% of the time. In May I also fueled at Apple Valley where I did my contest and in April I also got fuel from Monterey, Delano, and Bakersfield. It's pretty unlikely that any of these places had contaminated fuel.
In any case, I'm glad I did not need to send the tanks out to be serviced. I will flush the rest of my fuel system, inspect the injectors, and from now on use a strainer when refueling. I'll also look into getting those flakes analyzed. That will tell a lot.