Hi Geofrey,
Thank you for reaching out to us. Unfortunately there is no way to fetch all sitelinks on both the campaign and ad-group level. The AdGroupFeedService will return a list of AdGroupFeeds, which is used to link a feed to an adgroup. Fields returned include feedId, adGroupId, matchingFunction, placeholderTypes, Status, baseCampaignId, and baseAdGroupId. Please let me know if you have further questions.
Thank you,