The Forest River Sierra 403RD typically uses a 50-gallon fresh water tank. While we do not have an exact direct replacement listed, we offer several fresh water tanks that may be suitable. Options include the 26-gallon Forest River RV Water Tank (WT3855) measuring 50 3/4" x 22 3/4" x 5 3/4" and the Forest River Water Tank #F100074643 with dimensions 45" x 30" x 6 1/2" that features a fill inlet on the end. Since these tanks vary in capacity and size from the original, it is important to verify your current tank’s dimensions and fittings before purchasing. You can explore available replacement tanks ranging from 47 to 55 gallons on our website. If you provide your current tank's exact measurements, we can assist you further in selecting the best fit.