Maybe try using PGPeakLimit (a very simple compressor/limiter),
followed by a multimeter tool like ParMeter or RMS Buddy
http://www.kvraudio.com/product/dfx_rms_buddy_by_smart_electronix/details

Not recommending the link above, and have not tried the above; it just popped up in a search. I like ParMeter, but using any tool that also measures in a way other than the current peak value on the VU meter may be of help.

Using the RMS value of each song over time would likely get them closer pretty quickly.
Play with a combination of the track volume and the boost in PGPeakLimit to get the RMS of each song fairly close without having to overly crunch the signal in PGPeakLimit.

What that actual RMS number should be depends on your recordings, genre, etc.
How close is 'close' depends on how high the target number is; the higher the number, the closer the range should be.

At -22 you could have songs with a range of 6 and it would probably pass.
At -12 you'd aim for more like 2 or 3 differential. I'm sure there is a mathematical explanation for this, that someone like Mac could supply, but just knowing these kinds of tools are available may be of help.

I do not work here, but the benefits are still awesome
Make your sound your own!