A.H.
I completely understand! I'm almost 14 weeks pregnant with my third biological child/4th child in the household, in addition to working and the fact that my husband just left for 10 weeks of work-related activities in another part of the country. I know exactly how frustrating the exhaustion is...when it seems like everything needs to be cleaned, but everything is just getting dirtier by the minute. I think it really gets you into a negative cycle when things get to that point.
Make sure you're drinking lots of water and snacking often on fresh fruits and vegetables. Exercise also really, really helped me out a lot. Even if you just go out for a walk every day, it really does help to boost your energy levels. It will be hard right at first, because you will feel like you don't have the energy to do it. But the first few times you just have to force yourself to do it knowing it will help you in the long run. Put the kids in the stroller and take them out for a walk in the middle of the day when it's the warmest...or load them up and drive to the nearest mall and just walk around the mall a bunch window shopping. I've always found that I tend to walk fast through the mall, backtrack to different stores, and back to wherever the elevator might happen to be, and I end up having sore legs that night and sometimes the morning after.
Also, have you made sure that you aren't anemic or just have a low iron count? If that's the case, the best iron supplement I have found is Floradix, a liquid supplement that you can find at Whole Foods Market. I do have a low iron count...not quite anemic, but low enough that it affects my energy levels (and, in addition, low enough in my case that it causes some dizziness). I get an increase in energy almost immediately when I take it.
Something else that has been reccomended to me and has worked very well in previous pregnancies for me is taking Royal Jelly. Of course, whether or not you would choose to take it depends on your views on taking bee products and honey-related products.
I edited because I wanted to add that having a family member or friend help out a bit really helps a lot. If you have someone that you could ask to come and help out, it would do you a lot of good. Or if there was a teen in your neighborhood (or family) that was interested in making a few extra bucks on the weekend to come over and help with some of the housecleaning, that would also help you out a lot.