If a person is talented or knows someone with a plumber's or oxy acetylene torch they can grind the place where a ground cable bolts to a trailer frame, shiny, cause it's solder time.
These are the lugs I use on serious vehicle systems...
https://www.delcity.net/store/Heavy!Duty-Die-Cast-Copper-Crimp-Lugs/p_1013.h_91785I use paste flux yeah plumber's paste flux from Home Depot in heavier wires for soldering heavier wire and lugs and all that have been done have not suffered since the days of "Hell No We Won't Go".
Bolt the lug tight against the frame. It takes some heating to get the 60/40 solder to flow. Don't char the cable insulation. Some triple wall heat shrink tubing, then double coat everything including the bolt head and nut with undercoating and you have a "forever connection". It takes a Kelvin Bridge 4 terminal milliohm meter to read resistance through the cable into the frame, all the way up to the other end of the rig and to the END of another cable that has been similarly attached. Yeah, even after 10 years at the sea shore the resistance reads the same. Oh lazy me, do it once, do it right and go on to other things.