3500W at 12V would pull 300A, that's a lot of power. Things will get hot if you plan on running that load for more than a few minutes at a time, not to mention your batteries depleting quickly. Typical RV installations use 4/0 wiring and a 400A slow blow fuse.
We have a Magnum 3000W inverter that I ran for an hour or two in 100+ weather this summer at about 200A (AC and other nominal draws) and noticed that even with 4/0 *short* wiring and sufficient battery capacity that it didn't like continual running at that level; we typically use for minutes not hours of high load (microwave, hair dryer, etc) and it's much happier.