Is it permissible to perform ghusl (ritual bathing) after a wet dream using tap water and washing all body parts by hand, with the intention, in the event that using a bathtub is not possible?

If you immerse your entire body in water with the intention of purifying yourself from ritual impurity, then your ghusl is valid. There is no difference between performing ghusl in a dedicated basin or elsewhere; the important thing is to cover the entire body with water.