This is how I do it.

/**
* Classifies an axis-aligned bounding box in relation to a given plane.
*
* \param _aAabb The AABB to test against the given plane.
* \param _pPlane The plane to test against the given AABB.
* \return Returns a plane classification.
*/
LSE_INLINE LSM_PLANE_INTERSECT LSE_CALL CClassify::Aabb( const CAabb &_aAabb, const CPlane3 &_pPlane ) {
// Center of the AABB.
CVector3 vC = (_aAabb.m_vMax + _aAabb.m_vMin) * LSM_HALF;
// Positive extents.
CVector3 vE = _aAabb.m_vMax - vC;
// Compute the projected interval radius of _aAabb onto L(t) = _aAabb.c + t * _pPlane.n.
LSREAL fR = vE[0] * CMathLib::Abs( _pPlane.n[0] ) +
vE[1] * CMathLib::Abs( _pPlane.n[1] ) +
vE[2] * CMathLib::Abs( _pPlane.n[2] );
// Distance from box center to plane.
LSREAL fS = _pPlane.n.Dot( vC ) - _pPlane.dist;
// If less than R, return overlap.
if ( CMathLib::Abs( fS ) <= fR ) { return LSM_PI_INTERSECT; }
// Otherwise it is in front or back of the plane.
return fS > fR ? LSM_PI_FRONT : LSM_PI_BACK;
}

/**
* Determines if the given AABB is partially or entirely inside the frustum.
*
* \param _aAabb The AABB to check for being partially or fully inside the frustum.
* \param _fFrustum The frustum.
* \return Returns true if the AABB is fully or partially inside the given frustum.
*/
LSE_INLINE LSBOOL LSE_CALL CTest::AabbFrustum( const CAabb &_aAabb, const CFrustum &_fFrustum ) {
if ( (CClassify::Aabb( _aAabb, _fFrustum[LSM_FP_LEFT] ) == LSM_PI_BACK)
|| (CClassify::Aabb( _aAabb, _fFrustum[LSM_FP_RIGHT] ) == LSM_PI_BACK)
|| (CClassify::Aabb( _aAabb, _fFrustum[LSM_FP_TOP] ) == LSM_PI_BACK)
|| (CClassify::Aabb( _aAabb, _fFrustum[LSM_FP_BOTTOM] ) == LSM_PI_BACK)
|| (CClassify::Aabb( _aAabb, _fFrustum[LSM_FP_NEAR] ) == LSM_PI_BACK)
|| (CClassify::Aabb( _aAabb, _fFrustum[LSM_FP_FAR] ) == LSM_PI_BACK) ) {
return false;
}
return true;
}

The frustum is made of planes and each plane is a CVector3 and float distance.

Don’t mix your types. For example, you are doing a dot product manually. Why? Use your primitive types everywhere so that what you are doing is not only clearer but easier and faster to code.

L. Spiro