Just to confuse the issue, some things that used to be options are now standard. A spare tire, for example, was an option before about 2002. Possibly the propane tanks (as opposed to the propane in them). And I think the battery. In order to account for the bulk of the change, some research in this direction is needed.
I'm under the impression (not confirmed in any way) that equivalent models weigh about the same as they always did. In other words, when TM designers added something, they found a way to take out a more or less equal amount of weight.
Bill
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