We have a 2000 sf double wide mobile home that we bought new back in 1999. It came with a brand new Intertherm 3 ton unit. We live in South Carolina and our house sits in the middle of an old farmer's field, NO SHADE on the house from dawn till dusk.
We keep the thermostat set on 78* ALL the time. The unit will run ALL day, sometimes turning off, keeping the house between 78* and 80*. But on days like today that it was 90*+ degrees it run constinetly and the temperature in the house can get up to 83*. At night we don't have this problem it cuts on and off like it should. My question is, is this normal for an almost 10 year old unit or is there something wrong with it? If it is a problem, what does it need?

It should run just about as good as when it was new if it has been maintained.
When was the last time you had a professional look at your system and check the pressures etc?

Air filter clean?
All registers open?

If so ten you have done just about all you can do.

The outside unit might need a coil cleaning as well as the inside coil.

We haven't had a service person look at it in awhile. Hubby changes the filters regularly. We have registers opened in the rooms that we use (all except the spare bedroom and bathroom). Hubby cleaned inside coils beginning of last summer but didn't do outside ones. We do hose clean the outside every now and then but don't "clean" it with any cleaners.

You might try cleaning the outside coils WELL. Flush from the inside to the outside is what I do. Naturally turn off the power first.

What kind of cleaner do you recommend or will any coil cleaner work?

For outside they make a acid based cleaner. Read the bottle wear gloves. It helps to foam the dirt out. You will have to check around to find it. Use a pump sprayer to get it into the coils.

If not then you can use any good quality cleaner buy a pump spray to get it into the coils.

Thank you so much. I will have hubby pick some up this week and see if that helps any.
Have a great night.