Okay so I've been thinking. If the water heater is 240v because it has backup elements but you've got it set not to use them, and also it helps make the motor a smidge more efficient at higher voltage, but the motor itself doesn't actually draw that much power, then there's a chance you could run the thing from a modest-sized 120v generator with a step-up transformer. These are available fairly cheap, like $80 for a unit rated at "2000 watts" but I'll eat my hat if it can do that continuously. Multiply by the Amazon fraud factor and it's probably a 1000-watt unit, which is still probably plenty for just the motor.

You can measure this. Get a clamp-on ammeter and clamp it around one leg of the waterheater's 240v service while it's running. Multiply the reading times 240 to get the wattage. Do this right away and use it to inform your generator shopping.

If it's under 5 amps at 240v (1200 watts), I would consider that practical to run from extension cords. You'll need to do some wiring to add a plug since I assume the unit is hard-wired, and although you've got the backup elements disabled it still has thickass wire to accommodate them. I would consider disconnecting them entirely for safety's sake, then you could put a normal NEMA 6-15 plug on it, and a corresponding receptacle on the house, and just plug it in when not running from generator. When you are running from generator, the 6-15 will fit the socket on the front of the step-up transformer, then just run a heavy (12AWG or 10AWG) cord from the genny to the trafo.

(Yes you could get lower loss by putting the trafo right next to the generator, so the cord is carrying the higher voltage at the lower current, but then you'd either be putting the wrong plugs on things so you can use standard cords, or equipping a cord with 6-15 plugs which makes it a single-purpose unicorn. Which may be fine, tbh.)

Use that same clamp-on ammeter (with a line splitter ) to measure the pellet grill and whatever else you're curious about. (This basically turns it into a plug-in watt meter which may be a good investment anyway.)