Jack, You DON'T own the cabin in the sense of "fee-title". You may own the improvement residing ON the land but once the lease is up you'll need to get that cabin off the land. The owner is NOT required to renew the lease. In Northern California we have small communites which where built on BLM (Bureau of Land Management) Land back at the turn of the century with 100 year leases. They are all expiring now and the improvements are being removed due to ecologically sensitive areas. The families who enjoyed these properties had a sweet deal with very minimal "rent" for the land space over the past 100 years. The heirs are all raising heck but to no avail. Your land lease may also specifiy any improvments made to the land are the property of the land owner who was leasing you the property. I would have a real estate attorney go over the lease with a fine tooth comb!! Good luck!
You own a "movable" which means you leased a space for a temporary structure for a fixed length of time. The only "right" you have was to occupy a space for that period of time. You have no rights to occupy it indefinitely.