water tank placement and ground clearance

I am designing the placement of a water tank under the cabin. I realized I have no idea how much ground clearance one should have.
How far below the trailer frame can one safely go? I have seen some builds with foot wells. However, I have not seen measurements on depth of the wells or ground clearance after assembly. Most photos make it difficult to determine actual dimensions or clearance. Or the tanks are on massive trailers with 5+" frames.
Other constraints:
(1) I anticipate no real off roading (at least not intentionally), but will travel unimproved roads.
(2) Not having a trailer in front of me, I am estimating about 19" give or take a few inches from the ground to the bottom of the frame assuming 15" rims (28" diameter wheel) plus axle support. (technically does not matter)
(3) As the tanks get bigger, the minimum height is 10".
(4) Using the 3" frame and possibly some of the floor thickness ( 1/2" + 1 1/2 insulation + 1/2" ), one could get back up 5". So dealing with 5+" below the frame.
Also, any suggestions on best method to attach a water tank -- metal bands/strapping with or without a wood or metal base; metal framing (1" tubing); threaded rods with metal bars/strapping/bands.
How far below the trailer frame can one safely go? I have seen some builds with foot wells. However, I have not seen measurements on depth of the wells or ground clearance after assembly. Most photos make it difficult to determine actual dimensions or clearance. Or the tanks are on massive trailers with 5+" frames.
Other constraints:
(1) I anticipate no real off roading (at least not intentionally), but will travel unimproved roads.
(2) Not having a trailer in front of me, I am estimating about 19" give or take a few inches from the ground to the bottom of the frame assuming 15" rims (28" diameter wheel) plus axle support. (technically does not matter)
(3) As the tanks get bigger, the minimum height is 10".
(4) Using the 3" frame and possibly some of the floor thickness ( 1/2" + 1 1/2 insulation + 1/2" ), one could get back up 5". So dealing with 5+" below the frame.
Also, any suggestions on best method to attach a water tank -- metal bands/strapping with or without a wood or metal base; metal framing (1" tubing); threaded rods with metal bars/strapping/bands.