To be clear, I don't suspect that putting in the 12 volt fuse would make any changes. While I know the Sportsmobile manual seems to say that a blown fuse that no charge will go from the alternator to the aux battery. There is no
documentation from Surepower that supports this. Pulling the fuse, will disable the start assist feature of the Surepower unit. This circuit is listed as optional in the Surepower documentation.
To be honest, this feature has caused more problems than it has fixed. This is probably why the original owner pulled the fuse. If you really think you need a start assist feature it is best to make sure that it is wired through a switch (preferably momentary).
The Surepower 1315-200 can be divided into two major parts, the control module ( the part that monitors the voltage on both Aux and Starting batteries) and the high power solenoid or contactor. The primary contactor they use is the Trombetta Bear-DC-Contactor-Family Part #114-1211-020. Either of the two modules could fail, but the Contactor has the higher failure rate by far. You should be able to monitor the voltage and see if the control unit is telling the contactor to close at the applicable voltages. When the contactor does close you should be able to hear it. You can also measure the voltage across the coil terminals
These are shown in the picture by the yellow & black lines in the above picture. The most common failure in most solenoids is the that it get's stuck in the closed position, the most common failure in this application is that the internal contacts develop resistance overtime. It will appear that the Contactor will close, but there will be no or limited current going across the terminals. This problem is common to high current contactors and is exasperated when the switching current is much lower than the rated current of the solenoid. We have a 225 amp contactor that closes with much lower currents, The closing with lower currents tends to pit and make the internal contacts dirty, higher closing currents actually aid in keeping these contacts clean.
If you are having problems with the alternator not charging the house battery, the primary culprit on out vans has been the internal resistance issue. If you choose to stay with the Surpower unit, you can just purchase and replace the Trombetta Contactor, The best price I have seen is at MurCal Trombetta 114-1211-020 DC Contactor
I have seen some websites say they sell an upgraded version, part # 114 1211-010, this is the same unit with copy terminals, the -020 unit uses Silver Alloy terminals which is actually better for contact resistance problem.
There are also other options out there to replace the surepower unit with, you can search and come discussions on that. Most people have switched over to one of the Blue Sea units (I believe this includes Sportsmobile also)