i have a new 15 gallon holding tank which i did not use. it fits under the port settee on the 27 and may fit others as well. it is not an exact replacement tank but fits in the same place. i ordered it from tank depot and the part number is RH-B215. i will pay for shipping any where in the lower 48.
$125. the tank was $144 and the shipping was $52. i have had it about 6 months. we installed our new tank behind the head instead of using this tank. you will need to install your own access port and fittings. these are available from tank depot, wm etc. thanks