Im a little worried by the other LEO's chiming in on this stating a legal authority to perform a non-consentual search on private party while engaged in lawful activity and threatening 148 for walking away. There was no crime committed leading up to the contact therefor it is a consentual encounter and the OP has no duty to comply with the deputies request. For 148 to occur the party must be legally detained for investigation or under arrest and as it was stated the Deputy can not be doing an E-Check on private property as the weapon is not in a public venue or roadway. Some of you need a search and seizure refresher course! No wonder so many people hate us! Im sure Im gonna get flamed for this comment so go ahead and flame on.
So to answer your question OP yes you were well within your rights and I would have done the same. Politefully decline and go about your business.