Hi there, I own an Epiphone E-500MNS (Masterbilt series) and I've owned it for around 2 and half years. It's an amazing guitar and have no complaints other than over the last few months it's slowly becoming harder to keep it in tune.

I've tried messing about with the tuning pegs although there isn't a lot that can be done there; I also tried different locking methods around the tuning pegs but it didn't seem to make a difference. I'm not sure as to what to look at next and for the time being I'm not able to take it to someone to have it looked at. It stays in tune when I change the strings for a week or so, but after it begins to go out of tune after nearly every song I play. I use .11-.12 gauge strings if that makes any difference.

Is anybody able to help me out on the issue and has anybody come across similar problems?

Thanks in advance.
If you apply the string properly to the tuning machine's post, slippage should not be possible. So first... Check out the re-stringing guide at Frets.com and see if you might be having some problem in that regard.
Next... How old are your strings? A common problem with worn strings is that they won't stay in tune very well.
Next.. Are the strings binding in the nut? This can lead to an apparently in-tune condition on the tuner, then a minute later when the string "releases" it's out.
When you change strings, put a little graphite in the slots. Have you gone up or down in string gauge? This can cause the same problem...You might need to have the string slots "cleaned up" by a tech with the proper nut files.
Just a few suggestions....
First of all it is an acoustic guitar so all acoustic guitars have tuning and intonation problems. When you string the instrument the strings pull on the neck and on the top of the guitar. Look at your guitar from the side, how much belly does the top have? Over Time, that belly will grow, it will modify shape with playing, string gauge, and humidity, not only from the air but from you too. the neck bending can be fixed from the truss rod, but that doesn't change the fact that your guitar will bend like a bow. When you change the strings, the guitar will be without tension, so it will tend to return to it's original form. When you string it again, the strings gradually pull on the guitar again and the tuning, slowly becomes off. Lighter gauges may cause less tuning problems, because the string does not pull on the guitar so much. The pulling on the top of the guitar can be avoided by mounting a tailpiece. The intonation can be solved by filing the saddle and the nut to change string length, but this has to periodically change so the best solution would be to mount an electric guitar bridge, a fixed Fender bridge would work. I did all these on my guitar so I can tell you for sure that they work.
Although bloodfont's ideas may work, they are extreme and I can assure you that many thousands of acoustic guitar players get along just fine without such heroics.

Do the simple stuff first. New strings, and have a tech make sure your nut slots are in good shape. If the guitar has a cheap plastic nut, you might want to consider having a bone nut installed. Doesn't cost much.
Read the re-stringing guide I linked to or just have the tech install the new strings.
Intonation is either good or not. Nothing to do with the strings will have any effect; intonation is a result of having the frets properly placed (a given with modern guitars) and the saddle properly adjustted/compensated.
Most all factory-made guitars from decent makers are going to have the intonation set up properly, or so close that normal human beings without uber-hearing can't tell the difference.
Even severe warpage of the neck would have little effect on intonation... It's a matter of string length.