I thought that God becomes present in the tabernacle when the priest consecrates the bread and wine. This is called the “transubstantiation,” when during the Sacrament of the Eucharist the substance of bread and wine do not remain, but the entire substance of bread is changed into the body of Christ, and the entire substance of wine is changed into His blood. If there is no transubstantiated bread and wine, God is not in the tabernacle and having ones back towards the tabernacle should not be a problem.
Transubstantiation occurs with the bread and wine presented on the altar. The tabernacle is used for the reservation of the Eucharist remaining after the Mass.