2016 Winnebago Vista 30T

Lake Jumper, I looked under the back of the MH and there is plenty of room to mount a tire and wheel under there. I did that with the DP I just traded in after all these years. Easy to do and by mounting it with a boat trailer winch cable, you can lower it up and down without hurting yourself....I just have to figure out where to mount the winch.

Exactly what DaveinTN said. It is for the outside lock to the storage for underneath the master bed. I believe they used a separate key because someone could possibly gain access to the inside of the RV.
There is also a new Winnebago owners forum where you can post and find out about specific mods, issues, etc. Of course, this forum (RV.net) is my primary forum that has a wealth of info and discussion which is why I peruse both!
Here

Edit:
Trade winds, you must have been posting as I was typing! I can't imagine lifting a spare wheel/tire in that storage compartment but it's nice to know what it was designed for! Besides, my unit didn't come with a spare rim/tire. Did yours?

Dave, you are the winner.. It is for the outside storage compartment in the bedroom slide out that is set up to carry a spare tire.

Someone suggested I update my picture and MH info and as soon as I remember how to do that I will..... It was hard saying goodbye to the Tradewinds after over 150,000 miles but it was time...

My Vista 35B has separate key for an external storage compartment under the master bed, on the outside of the slide. It is a different key from the other external storage compartments. Do you have that?

Possibly a spare key to the previous owner's toad.

Pictures of the rig?

Could it be a key for a spare tire lock?

Might be an extra key for the slide outs.