Once you have this issue taken care of, a couple of other things to look at.
House batteries wired wrong, I suspect that this was dome when new batteries were installed. In any case one of the prescribed methods for wiring batteries in parallel is to have the ground on one end of the bank and the positive on the other end of the bank. Hopefully you can remove the positive cable and it will fit through the access hole and you can attach to the other battery. This distributes the load and charging batter.
I would check to see where the start assist wire is coming from. It appears to follow the cable from SurePower to the starting battery. I would suspect that they may be going to the ignition pin on the vehicle interface connector.
Pin 6 - Ignition signal is protected by a 10A fuse.
It is +12V active at ignition positions: Accessory
(1) and Run (2). It is not active at Ignition OFF (0)
or Crank (3). Whilst it can drive equipment directly
it is recommended to use this feed to control a
converter fitted relay, especially for high current
If so that would be wrong. As you can see from the Ford BEMM it would be HOT +12 Active when the key was in accessory or run, but not when the engine is cranking or starting.
Sportsmobile has a history of getting this input wrong, so it would be worth checking out. There are really no pins on the vehicle interface connector that support the start assist feature. You could test this feature, by checking with a meter, if it is HOT +12 active when the vehicle is running, I would pull the fuse.
Why that matters
It is very possible that the start assist feature will override the built in voltage monitoring of the Surepower unit, If the alternator was to fail to keep up with load on the house side, it would not be able to disconnect as designed if the voltage was dropping.
The larger fuse feeding the Surepower main terminal is actually in a brain dead position. The fuse is most like to large to protect the Surepower unit. I am not opposed to fusing from the start battery, but why not do it closer to the actually power source. Even more specifically, before you put it through a metal floor. For that matter, I would have just put the Surepower under the seat.
To protect the surepower from becoming a flame thrower if something goes wrong it is recommended to put a 10 amp fuse on the ground terminal connection. Any fusing on the larger battery inputs would be to large to protect the voltage monitoring module.