Why am I being taxed on my camper?

I purchased a camper a few years ago and put it on a campground lot which I rent. It's been there the entire time. I do not register it for the road or anything; it stays on the lot year round. However, the town office lady told me that I need to let the assessor know that I paid excise tax last year and fill out a form for the assessor so I don't get charged property tax on my camper. I filled this out and turned it in on April 15th. The lady told me I got the assessors paper in on time and to show up before May the 1st to pay my excise tax for this year. I now just showed up to the town office again today to pay my excise tax before May the 1st (today is April 27th). I spoke to a different lady, who is also the owner of the campground, and she told me that I came to late and that I was supposed to have paid my excise tax by April the 1st. She told me also that I am now going to get a bill in the mail for property tax verses excise tax. That is over $100. I don't understand why I am being charged tax's on a camper I own and don't run on the road to begin with. Please help. I live in Maine. If you need more information please let me know?