As others have said it is normal for the Surepower Separator to generate heat, due to the continuous duty solenoid. Other issues that could add to that heat would be loose terminals, not fully torqued terminals will add resistance and generate heat. Also there can be contact resistance that is built up on the internal terminals, this also will generate additional heat.
If your solar controller is in "float" with a reading of 13.5 volts and 3.5 amps, I wouldn't suspect either of those issue, except it is hard to tell with solar, as it could be limited from sunlight input.
Also as others have said Blue Sea Systems offer a couple ACR's that are not continuous duty. The 7610/7611 are both "solid state" or FET based units. I don't generally recommend these units with alternators higher than 120 amps. Also if you were to change with existing wiring which i expect 2 awg, than the unit would be derated for 100 amps . ( This is because of heat transferred from cable to FETS). These units use 175mA while closed and 15mA while open
The other units commonly used are 7620/7622, these units use a magnetic latch relay, still used one coil, but based on polarity at the coil a magnetic latch will hold open or release plunger. The 7622 adds a manual knob, that you can change state even if the unit has no power. The 7620/7622 exceeds the current specification of the existing Surepower 1315-200. The 762X series uses 13mA both open and closed, but does use 7A when changing state.
If transitioning to either of these units the terminals on the existing cables would most likely need to be replaced as the bluesea terminals size is 3/8 while existing surepower was 5/16.
I have a spreadsheet with the comparison of the bluesea units
You can also check out this from bluesea ChooseACR