In the key of D major what is V65/III but, the III is natural? I'm having a hard time with this. Also, V7/bVI?
In D major, III is an F♯ major chord. (It isn't diatonic: iii or F♯ minor is.) That being the case, V65/III is a first inversion dominant seventh chord on C♯ (see Ex. 1). However, V65/III is the same chord as V65/iii, so it isn't that far removed from the key as secondary sevenths go.
♭VI in D major is going to be a B♭ major chord. It's pretty much a borrowing from the parallel minor key (D minor). That being the case, then V7/♭VI is going to be a dominant seventh on F (see Ex. 2).
I'm actually going to have to disagree with the first half of Patrx2's answer. The second half is totally fine; ♭VI in D major is B♭, so the V7/♭VI will in fact be
F A C E♭.
As for the first one, OP asked:
In the key of D major what is V65/III but, the III is natural?
Since the III is both uppercase (thus not iii, which would be F♯ minor) and natural, it seems the question is asking for the V65/F (F being the "naturalled" [that is, lowered] third scale degree). The V7 of F is a C7 chord,
C E G B♭, which will have the
E in the bass for its 65 position.
In D major III is an F#major chord. But you say it's a natural, so I think we're talking about F major (which would normally be labelled as bIII). So you're asking for the first inversion dominant 7th of F, which is C7/E (C7 with E as the lowest note).
bVI in D is Bb. The dominant of Bb is F.
The third note of the D major scale is F#. The minor third is F natural. But we call the F chord bIII, even though 'flattening' it resulted in a natural.
In the same way, consider F major. Fourth note is Bb. So a Bb chord is IV. We call a B (natural) chord #IV, even though the actual note is a natural. The Bb has been 'sharpened' to Bnat.