Currently, custom fitting installation on RV fresh water tanks like the 36 gallon W1398 is not a standard public service, but we can explore the possibility by engaging our technician for a limited time. To proceed, we require a detailed drawing of your tank with precise measurements and the exact fittings you need, including their placement locations. Based on your description, you want a 1/2" NPT female fitting at the top left corner, a 1-3/8" barb fitting near the top on the left lower side, and another 1/2" NPT female fitting at the bottom center edge, along with level sensor installations. Once you provide these details, we will forward them to our spin welding department to evaluate and perform the custom installation. Please submit your tank diagram and fitting specs to move forward with this service.