It sounds to me like you are being used for convenience reasons.I agree with curlysue.I know it is not easy to move on...but trust me it gets better.

You do need to make a stand, or move on. It's crunch time for you but they are blissfully unaware because you, same way I did, allowed them to be. It took me disengaging and DH seeing his 'cherubs' for who they really are before things started to get better.
I see, pretty typically, that daddies will bury their heads in the sand on a lot of issues. As long as things do not bother them, or seem to bother others, then they do not make waves. They try to go into denial and ask 'what's the problem?' when they know very well what is going on. Tell DH that you really do not want to insult his intelligence, as long as he does not try to insult yours...I did that and he quit acting like he was a dumbass.
How to raise awareness is key, dropping the bugs in the right ears, making it look like their idea and all of the things women do for self preservation when husbands are in denial. One of the best ways to disengage is to withdraw access to funding! Can you afford to run the house on your income? I am guessing you can, since you managed before this all started. It could be well worth just maintaining basic utilities and just having bread, milk, potatoes, eggs and cheese in the house, better yet Ramen noodles! If you feel like something a little different then arrange to eat out with a friend from work. Let him know the 'pinch' of him not contributing, whether or not it's impacting you! There could be dollar store items for laundry, bathroom products etc. and you SAVE big time and set the money aside in a fund that only you can access. You can keep your own products for yourself or at a friend's place and do laundry there, take luxurious baths or showers same place.
When it comes to the SKids visits then they have to make do with meagre supplies and you being absent, take some time out just for YOU when they are visiting, especially if it's unannounced!
You are a woman in your own right, you are not just wife, mother, grandmother and step mother...seriously!
I cannot make the choices for you, I am not in control of other people and neither are you. You are in control of what you do though, as I am, and my suggestion is to take back the control you have allowed to slide, I did it too, and watch how quickly things change for the better. A good place to start is taking a good look at your budget, look at how his debt or bankrupt status impacts you. I sincerely hope they cannot garnish your income?! What we had to deal with was back child support that should have been paid out of household income...BM did NOT pay it in spite of having hundreds of thousands going into the house from DH! I sucked that one up because ultimately DH was liable for it and we could afford to repay it. I did not want for anything, though, so no hardship there.
Look at his relationship with his kids, is he dumping them off onto you? If so then you are 'busy' when they visit. If they want anything from you then tell them to 'ask dad'. Be nice to them, though, when they complain about no sodas or treats then tell them politely that money is tight right now. They have the pre requisite food shelter and clothing so no comebacks there! So what if you are percieved as evil...LOL you and I both know better...right?
The runaway? I believe there's absolutely nothing you can do about that except wait until the kid returns. I would not waste recourses on looking for him because he's probably with a friend right now. You will only hear from him when he wears out his welcome at said friends' house. If something serously untoward happens then you will be notified, truly, the authorities cannot do a damned thing because he is of age and has choices.
Think back to the man you met and married, how much have both of you changed over the four years? I read somewhere that it takes, on average, seven years for a family to 'blend' and I was not willing to wait that long for SKids to get their act together and I disengaged. I love being married to DH, he is a wonderful man. It is HIS responsibility to deal with SKids, since no one else in our house likes my style of parenting or honesty.
I wish you well, if bad feelings still keep dogging you then please see a counsellor. It was a great counsellor that told me a few truths, court ordered because SD is THE worst category juvenile delinquent in the entire state! The system looked at us as parents and evaluated us...we came out squeaky clean and the kid is a mess regardless of what anyone does. I listened to the counsellor's words, she said ''they are going to grow up regardless of what you say or do'' and advised me not to waste my energy trying to effect changes...she's absolutely correct! I disengaged, not in a negative sense, I did not fall out with anyone. I just stopped enabling them to be brats towards me!

Cut the cord and move on with your life !!!!

He's a mooch and taking advantage. I think you know your answer, its just hard to let go. ((((((hugs))))))