Hi, Civil War PhD candidate here!
Yes, southern soldiers brought enslaved African Americans with them as body servants during the Mexican-American War. The US government sometimes compensated camp slaves with pensions for their wartime services. None other than the commander of the US Army and 12th President of the USA, Zachary Taylor, brought a slave with him to war. The Army did not keep records of slave deaths, but soldiers’ letters suggest black slaves (who were often not given adequate supply and shelter) suffered greatly in camps. However, interracial socialization did occur in the camps, especially when it came to two of the soldiers’ favorite pastimes: drinking and gambling. Enslaved blacks also held fandangos with local women, who were rumored to have preferred black men. Many slaves also left the camps and escaped to Mexico, which had abolished slavery [see https://books.google.com/books?id=7QZoSoy_2DYC&pg=PA480&lpg=PA480&dq=camp+slaves+army+mexico&source=bl&ots=hiPGWIxOOi&sig=_sFIhmudhCN5vp93ScHrBfWdDlU&hl=en&sa=X&ved=2ahUKEwjq54Th6erYAhVJ-qwKHdF1Ac4Q6AEwBnoECAcQAQ#v=onepage&q=camp%20slaves%20army%20mexico&f=false]
The use of camp slaves was pretty widespread before the Civil War. In fact, Dred Scott (for whom the famous case was named after) was a slave who was transported to Minnesota by his white southern owner. Slave owners transported their slaves throughout the West before, during, and after the Mexican-American War, even taking them as far as Oregon and Washington [see https://www.nps.gov/articles/aslavefreed.htm].
Free black Americans also enlisted in the military during the war, despite an 1821 order excluding them from military service [see http://www.latinamericanstudies.org/mex-war/Blacks-Mexican-War.pdf]
Side note: In “Gone with the Wind,” Margaret Mitchell describes a scene where Scarlett O’Hara meets a slave named “Old Peter” who was a camp slave during the Mexican-American War. Mitchell’s grandfather fought in Hood’s Texas Brigade during the Civil War, and their family had a history of bringing slaves with them when they went to war.