Single Family's are tough Ive never got into them, difficult to cash flow and when vacant you have nothing coming in at ALL....especially with debt. I read several of the comments and I didn't see one suggestion. Assuming you don't want to sell these and move to Multi-Units you have to ask one major question.
Why are your tenants turning over so often? Is the location just transient? Are you asking too much? Are the utilities too high?
Quick turnovers will kill a lot of good deals. 300 cashflow isn't enough unless that includes a large repair % you are taking out every month. What are your numbers? How much to do you take out for vacant % repairs ECT ?
Also finding the right materials will help you big time, buying the cheaper flooring and even paint will only bite you in the but down the road. I have hardwood floors and they stand up to beatings. Vynal sheeting and quality tiles will last FOREVER if installed correctly.
Quality paint and painting every house exactly the same will help cut down costs then you can buy in bulk ( 5 gallon buckets ) you can go with white but I feel that's lame. Find the colors you like and stick to it ...forever. I found a few differnt grays with white trim works for us the neutral color appeals to most everyone. Also make sure you pick a brand that's going to be around I use beher from home-depot but sherman williams prob be fine... the main point here is so that you can buy in bulk and store it properly so when a turn over comes up you just grab it and go and you can match the old stuff on the walls... then you can just paint or ( have the painter) roll out the blemishes and in SOME cases not even have them cut in get a discount lol I know this sounds cheap but when you find the right colors no one notices. hope this helps !