AFAIK a trailer connector does have a ground lead from the tow vehicle, no?
And I'm not against renogy per se - I got a voyager SCC for my starting batteries. It's just that there are a lot of complaints from customers with failed units and inability to get competent tech support. Unfortunately a lot of stuff out of china is like this - you buy it cheap, use it till it breaks, and if it does or doesn't work right, you throw it away and buy something else. I have no problem with that for cheap stuff, but when you get over $100 then I really don't want to have to replace it on my dime.
Just so you know, there are a lot of complaints about the Orion running really hot. Even the manual warns to install it in an open air area. I also think the wire gauge recommendation is based on the fact that most rigs will have the house battery some distance from the starting battery, and they make the same recommendation for ALL of their units. Since mine is a foot from each, and I had 10awg wire on hand, that's what it got. I did realize after the fact to also reduce the fuse size from 60 to 40 on my 18A charger. They can also put out a little more than their rated - not sure why, I see closer to 20A on mine at full tilt, so they may be adding some extra current carrying capacity to their wiring rec.
In summary, I personally would not want to try to run a 40A charger from a truck battery to a trailer battery through the trailer wiring. If I really needed/wanted that much juice I would install a dedicated cable for it with heavier wire to carry that much current. And I would use 6 or 8G wire, 'isolated' as they call it, with its own pos and neg wires. 6G welding cable is super nice stuff, very flexible, let'er rip. YMMV.
I would say 8g is fine, but I don't think you can get welding cable in 8. 6G will give less voltage drop anyway. Connect with its own anderson plug.