The fuse slots on the converter panel are probably all good for about the same current, they are all "bussed" together ... (see note)
There IS a downside to your approach, the larger fuse will not individually protect the water pump, i.e. if it were to get stalled by frozen impeller, the fuse would not blow and the motor windings would overheat first. While not really so likely, have to mention as a point of full disclosure.
Similarly, same for protecting the heating devices, they will be allowed to draw current up to the fuse value, so if they were to short, get internally wet, whatever, they would be able to draw more current than intended in a failure mode.
Those things said, such failures are somewhat unlikely, and one could always add in-line fuses at the devices to provide individual current limiting protection.
Your approach WILL work to deliver power.
"NOTE" - some older converter panels had a few 12 Volt taps that were isolated, they would see the battery but not the converter output. The one we removed from our previous TC was such. There should be a wiring diagram on or in the converter (or check the manual) for such detail.
Again, not trying to discourage, just trying to be informative / diligent.