I agree 100% with adding 120VAC capability to an LP tank-type water heater with Direct Spark Ignition. The very least expensive type is LPG with "standing pilot" which is the kind you have to go outside and light. They use LP when not needed, and you won't stay in the habit of climbing in and out of the RV to light it and shut it off to meet your demands. They also blow out and leave you without hot water when you expected to have it. We had one and it was a pain. I might replace the tank in a DSI/Electric water heater. Any other kind and I'd opt up to have DSI/LP.
Who's selling replacement tanks for these now? I'd seen them on MarksRV but not recently...