I will tell you that although I am a plumber by profession, I have never done a food trailer/truck and am not quite at that point in my build. I will tell you that tankless electric water heaters suck a BOAT LOAD of power and I have not seen any that really hold up for very long. And the ones I deal with are name brand and very expensive. They are not the cheap deals on amazon. I only cringe at the thought of how long those would last. Propane, on the other hand does not cost a ton of money to operate. In the past they have not "technically" been as efficient as electric because some of the heat would get lost through exhaust gasses, but they still cost so much less to operate because the cost per joule (amount of heat energy) is so much cheaper as propane. Also consider that if you are running a generator, you will be taking some sort of fuel and changing its form of energy so many times (potential to heat and sound to mechanical to heat to electrical back to heat) that you lose so much efficiency throughout the whole process. With a propane, you just take the fuel and transfer it straight from potential energy to heating the water.
Rinnai has a really nice unit that is condensing (heats incoming water with hot exhaust gasses) and is 98% efficient! Very little operating cost and a great brand name heater.
I have never used Aquah brand, but i think Dr. of BBQ has used it and speaks highly of it. check out reese's p30 build and i think he has some good advice on there.
Again, my only experience has been residential and commercial plumbing, but maybe it'll help with some direction and thoughts. I will be doing more intensive research in the weeks to come, and will be updating my findings to RF.