Buy a used boat trailer for a 16-18' boat, put your own decking on it. Trailer would be made for water use so should be able to handle sitting around better. Most boat trailers in that size should be at least 3k lb rated or more.
Don't know where you live but a quick search on searchtempest.com might get what you want.
Here is a example - http://dothan.craigslist.org/boa/3647704759.html
Here is another http://bgky.craigslist.org/bfd/3637517731.html
I am not saying to use 33-37 inch grapplers on the the trailer, but I see these 10 inch or 14inch white rims with tiny tires on it and wonder when the balloon is going to pop.
Why not go with axles and wheels and tires that support a standard 6 lug 5.5 pattern and 265/70/16's or something.
I would feel much safer with a "standard" wheel and tire even if I was only pulling 500-1000 or more pounds on a small trailer.... than little latex balloon tires..... :-)
My Jeep trailer used to run the same Pro Comp 36 M/T's and 16" wheels that I have on the truck that way I had more spares for the trail. But the trailer was only a camp/utility trailer 5x8 for carrying tools and camping gear, it looked nice lifted as high as the truck and could tag along most of the trail but not all, the height also made it easier to keep the hitch flat, but on the highway at any speed over 60 it swayed like a kite in the wind. The new owner has 33's on it and lowered it 3" and it handles much better now.
Christopher have you thought of using sliding mounts for your fuel tank? A few inch's forward or back will make a big difference in your balance point and as you burn or add fuel you could adjust where the tank sits.
I've seen this done on agriculture trailer that runs a 600 gl tank and pumps, the owner put a hand crank with geared wheel and a geared track on the trailer to make his adjustments. You could do something simpler maybe metal slides and use a come along with attachment points at either end of the trailer. Use floor mounted clamps to secure the tank to the trailer floor.
The Gen set you want to be solid mounted and since it's weight wont change isnt a variable in the equation.
Mmm, center of gravity ... another reason to use smaller tires.
Sliding mounts make a lot of sense for the tank.
Sliding mounts does sound like an interesting method. However, I'm not sure how I can keep the tank from binding on the rails when trying to slide it. If I put the tank over the axle, it shouldn't matter as much anyway.
I'm planning on looking at an old boat trailer-turned-utility trailer this weekend. As of now I don't know what the weight capacity of it is, but that will be the first thing I look at. I'm figuring a boat trailer should have a decent capacity.
My 4x6 utility trailer runs LT265x75r16s...and has a cut down Dana 60 for an axle, and aluminum Dodge wheels. Its purpose built...purposely built to go anywhere I can take the truck. It probably has more miles on it and has seen more states than most trucks/owners on this forum.
Building a trailer isn't that bad...you can also find a local trailer builder and have them custom build the frame. I priced a custom trailer done that way once near Shreveport, LA...a 6x10, tandem, 7k pound GVW was something like $600?? Again, this is a backwoods builder, not a commercial guy.
If I were to do it again, I would keep an eye on Craigslist for a frame (no title), then use the materials to build it how you want it. Mobile home axles are cheap, and work great for a trailer intended to stay local (they don't balance very well)...I bought a pair of axles, springs, and six newer tires/rims for under $100. And then you have enough axle/tire/springs for 6,000 pounds (and they typically come with brakes).