Long hot showers in an RV? You have the same kind of issues as with batteries and an inverter, only now it is with water. I am going to assume that this luxury is limited to when you are hooked up to shore power and water, and possibly a fixed source of natural gas or propane. Doing this off grid would require big tanks, pumps, a large propane supply, space for a 40 gal. water heater, or possibly an on demand type.
I haven't done what you suggest myself, so I can't speak to it directly. If you haven't already, you may want to broaden this conversation to the Escapees forum. There are lots more people who visit there who might be able to give you input.
Assuming you are doing this with hookups I see some issues. The water pressure from a hose bib (or whatever) might not cut it. You might need an auxiliary pump and possibly a small accumulator tank to buffer the demand. If you go electric then I assume it has to be 220V and lots of current, otherwise it will take a long time to heat the water. You might have crossed the line into needing propane. I have heard mixed reviews for the on demand type heaters for RV use. Assuming a tank type any residential unit should do. I believe they have propane conversion kits. In my experience propane takes a smaller jet due to the higher pressure from the regulator. Some have used an air tank to pressurize the system instead of a pump. Some trucks have a ready supply of high pressure air as part of the braking system.
Be careful to consider the weight of all these things you are adding. I have no idea what your truck is rated for but you keep adding heavy stuff everywhere, like water and batteries, and your tools are probably pretty heavy as well.