You cannot do this conversion because the end reulst must allow all rows to be in the final data type (numeric). YOu could do something like
if isnumeric({TLORDER.BILL_NUMBER}) then tonumber({TLORDER.BILL_NUMBER}) else 0 but that will not help too much for your sorting...
My other suggestion did not work for you? It should.
If you add lead zeros to your text string and then make all the results the exact same length it will sort it numbers only first from lowest to hightest and then alpha.
Create a formula field called "SortOrder" as:
right("000000000000000" +{TLORDER.BILL_NUMBER},15)