We're planning on building a gooseneck w/ a 20' deck for my powerwashing company this winter. The front 4' is going to be enclosed for the 2 powerwashers and the chemical jugs. The back half is going to be for a 1000 gallon water tank. We planned on getting 2 7000lb axles with electric brakes. Its not going to have a dove tail since it will get used for seed pallets for the farm.

Is there anything you guys would suggest we add? Anything you wish you your trailer had?

Where will you be getting steel from? have you put a cost together for it? Just curious as I have thrown this idea around....

I would suggest centering the heavy load (the water) direcly over the axles. Don't put that load on the "back half". Further, I don't know how much weight the "chemical jugs" will represent, but I'd put that near the water tank. Essentially, you want all your variable weight (weight laods that go up and down with the consumption of the product) to be as near the center of the axle lines as possible to minimize the differences in handling as the loads change and shift.

Everything else can be estimated weight-wise and just try to balance them out front to back. Make sure not to overload the GN hitch weight rating; too much will grind your hitch to a pulp, no matter how much you grease it.

I havent checked steel prices yet, we have a metal show about 15 miles away though. The water tank will definatly be right over the axles, since that will be most of the weight when its full. The chemical jugs will be 2 55 gal drums.

Does anyone know where to get the trailers part of the hitch?

1000 gallons or water is 8350 pounds. See what everything else weighs. Might need to up the axle ratings a little.

Check out Northerntool.com They have a Bulldog gooseneck coupler and trailer blueprints that might be helpfull.
http://www.northerntool.com/shop/tools/category_towing+towing-couplers