I wonder if what's showed in this diagram could be a simple solution in an RV rather than the typical separate return line? Maybe use an on/off switch or timer switch? Pump would have to be rated for hot water use. Would also depend on how your piping is routed between HWT and faucets.
Have often thought a recirc line & pump would be a good idea. Even if not dry camping, would save on unnecessarily putting water into the grey tank if no full hookups available. In our case, I plan to remove the underbelly one day and would modify the piping as needed while in there.
