We can assist with creating a custom RV fresh water tank matching your specified dimensions of 45.5" length, 25" width, and 5.75" height. The 1/2" female threaded inlet, outlet, and vent ports are standard features we can accommodate. Regarding the lip on your existing tank used for bracket support, please note that including it may affect cost and lead time. If preferred, you can fabricate new brackets to eliminate this feature and potentially reduce these factors. Shipping to Alaska is available, though transit times and costs will reflect the destination. To provide an accurate quote and lead time, please confirm if you want the lip included or will use new brackets, and provide any additional details or photos if available.