First thing I would do is to leave the empty original battery boxes on the tongue with a few 10 gauge wires running into the trailer. Make it look like you just removed the batteries and took them onto the garage or something. As suggested I would also try building a compartment to hide the batteries. Just leave the inverter outside the compartment, maybe even mounted right to the side of it. And yes even though there is a cool factor in what you have done, I would selectively refrain from talking about the batteries being there in the first place. What I mean by that is if I was camping locally I would not mention it to anybody as they might actually recognize your trailer as the same one they see in their storage lot. But if I was camping hours away from where it's stored I would be more likely to mention the batteries as the chances of recognition are smaller.
Want to get really creative make your storage container with a hinged lid you can open for maintenance or whatever. Then build a removable tray on top of the batteries. Go buy a really cheap but unused sewer hose and store it in the compartment. Make up a batch of chocolate pudding and smear it in a few places on the hose and the tray and let it dry there. Don't over do the smearing or it will look false,
