Has anyone ever had this problem.....

Since both tanks held the same amount of liquid, I'd say that you have actually found the actual volume of the tanks.
Since the TT is new, there shouldn't be any solids built up in the black tank. Gray? Seriously doubt any build up there either.
Perhaps the TT has an optional waste tank option? The 40 gallons might be the options tank and you have the standard???? Just pecking at options here.
I would ask the dealer why your actual measurement differs by so much from the advertised capacities.

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The trailer was level without a doubt........

In 1983 I purchased a brand new 1984 Komfort TT that was to have 2 40gal tanks. They filled up too fast. I measured and the tanks were only 20gal ea. The dealer replaced with proper 40gal tanks. Factory's do make mistakes.

Can you measure the dimensions of the tanks?

Since it is bought NEW, I would take it back to the dealer and ASK them to check the tanks and get a true size on them. hey, ya never know :)

strange the tanks are supposed to be one size and he can only get 26 gals. into the black tank.

either unlevel??? when checking or someone screwed up on those tanks while installing.

Since new, ALL of this can be checked with the dealer. He is best to go there and get a real answer as to why. hmm

Only thing I can figure is that the trailer wasn't level so the tanks would not full completely. Other than that, I got nuttin'.

Many people new to RVing, try to use water, showers, etc. pretty much like at home and the tanks will fill quickly. We take short showers, dump lots of the grey water down the stool or even carry it to the campground bathroom. We even shorten up toothbrushing. We can then go a week if necessary even with smaller tanks than you have.

Tank is mounted wrong-at an angle that 15 gallons will not drain, or if TT was parked at such an angle that 15 gallons did not drain.

So what kind of RV is it?