Motorhome Installation and Amp Power Wire Selection

I am preparing to upgrade my factory Class A diesel pusher motorhome audio suite as it sounds horrific. Not looking for a powerhouse, just good clean accurate sound with more depth because right now the speakers sound like frying pans. To replace what I have currently, I have 4 Rockford Fosgate P1675 Punch 6.75-Inch 3-Way Coaxial Full-Range Speakers, a Kenwood eXcelon DMX958XR head unit, a Kenwood Excelon X302-4 Class D 600 Watts max and I am cheaping out on a temporary Rockville SS8P 400 Watt Slim Under-Seat Powered Car/Truck Subwoofer as I am concerned that this config may fall short on bass. It is only $100, I have spent my wad on everything else, don't judge lol. As everything is sitting in the corner waiting for installation, I am doing my wiring plan. I can't find anything on how to select power wire for a diesel pusher. Here is the issue. Motor and batteries in the back of my 36 Ft motorhome and the head unit and amp is in the front, Ahh the plot thickens... If the Amp and the sub will draw 90 Amps at max but I can't even find a table to calculate the gage. Having said this, I do have a curveball. There is a front chassis fuse panel that has two power leads chassis and house and as near I can tell the panel is also 90 Amps but I am not sure the factory wire could support 180 Amps in total. I don't even think 00 wire could support 180 Amps over that distance but if it could I would fuse and wire from the front panel with 8 or 10 gauge to the amp (3-5 feet). It would probably be easier and cheaper to just wire from the back to the front directly to the amp. But like I said, I cant figure out what gauge to use over 36-40 Ft to deal with the 90 Amp max draw. Thanks for your input!!