Washing a large dog without a bathtub can be easily done with a few simple steps. First, choose a suitable area for bathing, such as a spacious shower stall or an outdoor space with access to warm water. Use a handheld showerhead or a large container to wet your dog thoroughly, avoiding their ears and eyes. Apply dog shampoo, lather it gently into their coat, and then rinse thoroughly. Use a towel or a hairdryer on a low setting to dry them off, paying attention to their underbelly and paws. If needed, you can also use grooming wipes to freshen them up between baths. With a little creativity and adaptability, you can keep your large dog clean and fresh without a traditional bathtub.