Just because an alternator is rated at 140A, does not mean it's spinning at the right speed to generate 140A.
But yes, a immersion heater is a purely resistive load, and should not pose any problems for a mod sine or pure sine inverter, rated to deliver 130% of the load.
If your heater is 1500w, you inverter should be spec'd to about 2,000 w so it is less likely to over heat.
1500w is 125A, so that's a beefy load, and your battery/inverter cables need to be able to carry it without resistive losses.