You can buy an adapter to go from 240 to 120 but not sure how good they are for high use.
My wife bought a glue gun from the USA so I had to buy an adapter but it only lasted a year before the plastic case cracked so I through it away.
I may be wrong but I would think that the OP's fiver already has a transformer unless it is a new import.
Ours certainly had one fitted so that the fridge, microwave and electric fire worked, gas only boiler on ours, still working after 12 years of almost full timing.
Sorry no answer to original question.
If you need a step down transformer it will have to be fairly beefy to cope with the load but I doubt if the frequency difference between USA and UK will matter. Worth checking transformer in yours to see if it has any other tappings so it can be changed to 230v ( UK standard voltage)
If you can find an Atwood parts list you might get an answer there. At one time the answer would have been to simply change the element but these days it might have electronics supplied by a transformer that expects 110v. Why not ring Atwood? (I still think all it'll need is as you suggested - an element change - but you need to be certain.)
Atwood Marine 220 Volt Water Heater Element Atwood Marine Heater Heating Element 220v 1500w – Atwood Part Number 91581Also Known As Part Number 91851Heating Element With Gasket (Bolt On Type) To Suit All 220V Atwood Marine Water Heaters. Other Marine Water Heater Parts Available 80-8562...