Why not have a smaller battery pack, say something with 200-250mi unloaded range, and a small fuel generator with modest fuel tank to extend range, especially when under load?
For a long time, I've thought the same about an electric pickup. However, I recently watched a youtube video where the team used a Tesla X to tow a 5K trailer up the Ike Gauntlet. The on-board demand showed that the vehicle consumed 1.8Kw/mile. At 60 Mph, that would require just over a 100KW generator or 135HP to keep up. That's not exactly a small generator, and conditions would be much worse for a 14K trailer as being touted by Tesla. Granted, this is likely worst case, but does cause me pause when thinking of pulling a 10K travel trailer into a 40Mph headwind. I'm afraid it would quickly get into the same situation as the BMW Rex that doesn't have enough power to keep the vehicle moving. It would have to have a mode that enabled the generator to self charge the batteries when stopped (alongside the road even! )
Where I do think an electric pickup will shine is with contractors who stay "local" and don't take the vehicle on all day trips. Add an inverter to power the jobsite trailer from the batteries and you eliminate a portable generator. For these users, cutting the mileage by 1/3 down to 150-175 miles will still let them go all day and get back to the shop to charge. It may have to be coupled to an upgraded electric service as the charger would need to draw about 80 amps for 10 hours to recharge overnight, but if the shop is otherwise unused overnight it may still work out.
--Rick