The syntax of the group definition is incorrect.
Group diesel_group "Diesel" <house>
The square brackets in the docs mean, that these values are optional, you don’t write them in the .items file.
Second, you need to put the Diesel_* items into the group “diesel_group”.
Number Diesel_BN_JET "Diesel" (diesel_group)
Number Diesel_BN_AVIA "Diesel" (diesel_group)
The round brackets in an item definition line adds this item to a group.