There are a few credit repair companies around that can help you restore your credit score (CS). When all your bills are paid your CS score should increase however be sure to leave a little balance each month on a Credit Card to maintain that credit status and showing consistent payments.
As far as the rental gos you must show this on your tax returns. If it has been less then 1 yr you will need to show that they have been paying you. Cancelled checks would be best or a deposit into your acct... Cash does not work, so if they are paying you in cash start to have them pay with a Cashiers Check or a money Order.
Your late mortgage payment must be over a year to get a mortgage.
Your best bet is to have someone run your credit and advise you on what to do to bring your scores up. With a 30 day late, the passage of time will be an important component in that.