Thanks for the feedback, even those who questioned the junk trailer's legality (I don't care if it's legal).
I realize that 400-500 gallons of water don't go that far, but obviously it goes a heck of a lot farther than the 2.5 gallons in the Amerex water fire extinguisher that many of us carry. If 500 gallons is useless, why carry the Amerex? Right, it's useful, just very limited.
Besides, there's an irrigation canal that flows along the long side of my land which I could probably pump a
lot out of non-stop, or at the very least refill the 500 here and there, or the same out of my pond.
Finally, building and maintaining the fire trailer would keep Murphy at bay.... and besides, it would be a fun project, and from what I'm seeing around here, if I set up a good fire trailer, I could easily get my money back out of it if I decide to sell it.
About the junk trailer referenced above - while I think tying the main tubes together with cross-braces that would be necessary for carrying a tank would provide the main frame with sufficient strength (as mentioned in the original post), I think that the sketchy "axle" makes this trailer worth scrap; the frame itself is worth something but it probably needs an axle found. They're asking $200 on CL, so it may almost be worth that, but looking further afield I see some military M200 trailers at auction that would probably fit the bill really nicely:
The M200 trailer is a 2.5 ton, so they'd be right at "capacity" with a 500 gallon tank. As I previously stated, this trailer isn't going anywhere but on my land or immediately next to it, so I'm not concerned about the typical capacity concerns you have when sizing a trailer for a carrying a tractor, that's going to be registered, and towed on roads & highways -- this fire trailer, if it's actually made, will probably hit 15mph at max, so I'm really not concerned about loading it to its capacity (I'm not towing it behind an M35A2 that's bombing down a rutted dirt road at 35mph).
These trailers have air over hydraulic brakes, which obviously aren't going to be used when towed by either my truck or tractor. My tractor weighs about 5k; presumably pulling this in low or medium range will be somewhat controllable... my truck, which is the more likely tow vehicle of the fire trailer (since I'd prefer to have the tractor available for cutting fire lines especially if the fire is coming across the meadow like last year when my idiot neighbor decided to remove blackberry brambles with fire) is a heavy 1-ton and is definitely capable of handling this loaded at low speeds. If not,
something like this describes could be used to set up electric brakes.
(new 500 gallon poly tank from NT about $500; you could probably fit two 250 gallon IBC totes a lot cheaper)