Well, you can certainly charge them storage for any time AFTER you let them know there was a storage fee associated with keeping the rig on your property.
you mention you told them 6 months back...if you let them know 6 months ago there was a fee associated with future storage...you can demand they pay this amount for the time they stored the vehicle with you. (So, if you told them 6 months ago it was $100/mo. storage, you could charge them $600)
Now, as for storage fees for the time prior to you telling the lien holder of the fee, that will be difficult to enforce. Since you did not have a contract
with the lien holder, you will not be able to force them to pay in the same you you can AFTER you had informed the of the storage fee. You can certainly bill them, but if they refuse to pay, you will have a hard time convincing the court to force them to pay. This is true under the legal theory "privity of contract". More info here.
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