Tom is exactly right. A 240V 4500W(typical) heating element has about 12.8 ohms of resistance. The math says that same 12.8 ohms will use about 9.3A at 120V and deliver about 1125W. You will still get to full 120F or so tank temp, it will just take 4 times as long:)
You are off-grid and you are not harvesting engine coolant heat? I can get 3/4 GPM of 120F(with about 70F inlet) water out of my heat exchanger at a 3KW electrical load on my generator. 2KW gives me about 1/2GPM and 1KW around 1/4GPM. At 1/2GPM, it will take 100 minutes to replace all the water in a 50 gallon hot water tank with 120F water from the engine heatex... Below is a diagram of just such a system(mine), and it can be done with about $250 or so worth of parts and some misc plumbing and hose components. Depending on how you plumb in the radiator unit, you can either dump the excess heat, once the hot water tank is fully heated, into the home, or outside into the atmosphere if you don't need the heat inside. You are going to all the effort to process the fuel for use in the engine. you might as well get all the heat out of it that you can... Here is a basic parts list. The brazed flat plate heat exchangers will thermosiphon well, but it does require a little bit of vertical space to accomplish this. Mine is about 30" above the engine to get adequate flow at all load ranges...
600SQ/IN brazed flat plate heatexchanger with 1" NPT ports(E-bay woodfired boiler vender)
18" X 18" pressure rated coil radiator(E-bay wood fired boiler vender)
Small Taco bronze circ pump(E-bay plumbing vendor)
120F Thermostat from a Mercurty Mariner 9-14HP outboard motor( marine parts wharehouse)