Turn everything off, clean and tighten connections at battery and also ground connections to the chassis.
No problem with a solenoid having only one small terminal. That means it is screwed to metal and the solenoid exterior metal serves as ground.
Don't know where you are in Europe, but you should be able to find a 12 VDC CONSTANT DUTY solenoid of the proper amperage/ So, a starter solenoid is NOT what you need.
Another option is a marine ON/OFF switch with alternator and chassis battery to one lug and house battery to the other lug. Switch ON= combined. Switch OFF= batteries not combined.
