From what you have described it sounds like the starter solenoid is bad and that is the reason you have to hook the wire to the positive side of what looks like a Ford starter relay (which is probably a Ford starter solenoid).
As far as all of the positives and negatives being hooked to one battery there is no telling what is going on without looking at it. A lot of coach builders will run added on accessory wire connections to the chassis battery.
Jon & Sue Francis (Retired U.S.A.F.)
Lil Girl-Rescued, Abby Rescued, Peaches Rescued
06 Allegro 35TSA Workhorse Chassis
2013 Chevy Spark Dinghy