Happy John wrote:
One key, which we all carry, to fit multiple different locks. Of course, we'd have to rebuy the
locks when we sell the vehicle.

http://www.boltlock.com/

That is a great idea! Unfortunately, we don't even have keys for our
vehicles. The SUV and the convertible both use key less entry. There
is a backup key hidden in the wireless contraption to open the doors in
case of an electrical problem but they are cut on the inside.

My boat has a single key from Masterlock that takes care of the receiver
and hitch locks.