So to be clear the fuse in question is for the inverter/charger not the battery.
RVIA apparently does not demand that the Battery be fused as opposed ABYC. I would expect ABYC to be more strict, if the boat catches on fire you have a bigger problem if your RV catches on fire.
That said, I recommend that the battery be protected per ABYC standards, and run that on my rig.
This is the class T fuse I added on my rig. It is close to the battery
The inverter/charger is a power supply and thus is required by RVIA to have fuse protection within 18" The fuse is in a bidirectional circuit, and thus needs to handle the requirements of the inverter and the output of the charger. The inverter requirements demand more current so they are the driving force in the choice of circuit protection.
Sportmobile will fuse or add circuit protection for fuse panel , any device that draws current before the fuse panel should also have proper circuit protection.
In general fuses are to protect the wire, so anytime a branch circuit changes wire size there should be fuse that adequately protect the transition to a smaller size wire.
Of course your wire size should be defined by the current and voltage drop requirements of the loads they are going to.
On your solar install, you should fuse the wire from panels befote they go through metal of your van at a minimal. The charge controller should be fused close to it and then you can fuse the other end (Branch) appropriately if it is long enough and the wire changes size . So if you were running 2/0 gauge to the inverter and you tapped off of the class t fuse to go to solar with 6 gauge it would be proper to correctly fuse the output or the charge controller and the transition from 2/0 to 6 gauge. If the wire is short enough the one fuse could be good enough for both sides if that makes sense. Also I expect that the fuse branch connection would not be required by RVIA, but would bring additional safety, especially if it goes through metal, etc.
hope this helps,
-greg