If you can stay with a stock sized tank the $ will be a lot less. I was going to use a gray water tank that I scrounged five years ago, mounted below my floor, but I have a 4" dropped axle so ground clearance would be iffy. The revised plan is to have the gray water tank on board, above the floor. A bilge pump will take the shower water from below the pan to the shower-water-only holding tank. I know that sounds complicated but I will be having a stand-up shower in a low to the ground trailer and I wasn't willing to risk whacking something with the holding tank while going down the road....it would have been just in front of and lower than the drop axle beam, about 8" ground clearance. It would have been OK with a straight axle.
I purchased a couple of 16 gallon tanks off eBay as this size would fit under the beds. One will be used for fresh water and the other for used shower water. Originally I was going to carry 32 gallons fresh and use the belly tank, but I'm going to make a trade-off on this one. By having all the water on the inside of the trailer, freezing weather won't be as much of an issue, and I'll also be able to insulate the floor better in the area where the lower tank was going to be located.
There are several sizes to choose from, but as you do research on this you will find a wide disparity in pricing. You may pay dearly for some of the non-common sizes.
I have placed a couple of orders with these folks and they seem pretty good and ship quickly. Check their feedback. I've thus far talked with Paul, his wife, and his daughter...they are indeed a family operation, nice to see that these days.
http://cgi.ebay.com/ebaymotors/16-gallon-Fresh-water-tank-Camper-RV-Horse-Trailer-Boat_W0QQcmdZViewItemQQhashZitem56351850a1QQitemZ370257973409QQptZMotorsQ5fRVQ5fTrailerQ5fCamperQ5fPartsQ5fAccessories
Bruce