I am considering purchasing a used truck and having a motorhome built with an overall length of 45 feet. I am concerned about the front axle rating on most used trucks @ 12,000 pounds. I would like some input on whether that is a real issue or could I get by without a higher axle rating.

Overall length is a factor as well as wheel base. If you stretch the rear axle placement out to the far (rear) end of the frame, you might want to consider a 16K ft axle, as the weight will be forward biased.
My rig is 42' twin screw & has a 16k ft axle. This may be overkill however, that is how most of the "big boys" in Elkhart spec them out & I figured that maybe they knew what they were doing.

I know Show Hauler weighs each coach to make sure it is not oer on teh front axle rating. Mine is a 46ft unit with a twin screw and 14,000lb axle and we are just under the 14,000lbs when fully loaded with trailer.
So I agree with Bob, bigger is better.