The terms of an oral month to month lease can be changed with 30 days (one rental period) written notice. The notice should be provided in a manner than can be later proven if necessary. Post conspicuously w/ a picture dated of the posting or by certified mail return receipt requested AND first class mail. Retain copies of the correspondence.
I had a client with a similar situation. I suggested calculating an average of a years bills as a base amount the landlord would pay as included. Any overage would be allocated among the tenants. That way the landlord continues to honor the spirit of "all bills paid" while holding the tenants responsible for keeping them reasonable. As a practical matter, when one of the tenants sees another wasting energy that they will be paying for they will police themselves.
If there is no written lease, and the tenants are on a month to month tenancy, you can provide 30 days' advance written notice of a rent increase to cover the utilities. Make sure that you record how and where you have provided the rent change notice to each tenant. As long as you provide 30 days' advance written notice, you can change the lease and rental terms to suit the situation so that you collect the appropriate amount of rent.
Since you are in the Dallas area, you may wish to take advantage of my free hour of consultation for new clients. If you make an appointment, please bring copies of your pertinent paperwork.