Re: Some electrical questions, if I may...
Texas, I agree that keeping the AC and DC separate is better. However I could find no place in the manual to hook the returns from the 12V devices. Could set one up I guess to go directly to the battery negative, but would be additional work.
Rob, your proposed layout sounds ok to me, although I would put all the internal outlets (except possibly one which was NOT GFI) on the same breaker. But you can split the sides if you like, I just don't see any benefit to doing so. You can put the external outlets on a separate breaker so you can turn them off when not in use.
As long as the 12v fuses in the converter are appropriate sizes, there is no real need for 'inline' fuses between the panel and the devices. If you have several devices running from 1 fuse, then it can be useful to then have smaller fuses for each device, close to the device or at a second 'fuse' location for ease of service. Just keep this fact in mind when you go to troubleshoot a device which has stopped working.
Yes, you can use the frame for your 12v return/ground. Every car on the road does
Some electronics may have problems with noise introduced by having 120v AC grounded there as well. Another option is to use actual wires to get back to the battery negative (directly or through the converter panel). More costly and more work, but 'better'.
As I say, I could find no place in the manual where it indicates where to hook the 12v returns. I would be leary of sticking wires in the same hole where the battery negative is attached, but there does appear to be a spare hole in that connector which could be used. I wouldn't 'twist wires together' and use that hole; a couple of big wires together would work, but a bunch of little ones might have one pop loose, or worse, have intermittant contact. I'd use a big wire to a separate ground bus or termanal strip, then hook the individual wires to that.
The battery you have now may work adequately for a while, so trashing it and replacing it with 2 deep cycle batteries may be overkill. On the other hand, getting another just like it might be wasteful unless they are real cheap. If you do go with 2 batteries, they need to be as similar as practical (both electrical characteristics and age). In any case, I would find the deep cycle batteries I might eventually use, and build the battery compartment to handle them, so I could go to them at some time in the future when my needs increased or the old one(s) wear out.
Keep in mind that deep cycle batteries are available in 12v, so you would put 2 in parallel, but the 'best' ones are 6v, so you would need to put the 2 in series. Use BIG wires between the batteries, like the cables going to the battery in your truck.
