Great minds think alike! I was thinking of a hot-water recirculation system from near each hot spigot back to the heater to save waiting-for-it-to-get-warm water when boondocking.
I hadn't thought of a convection system. That would probably work fine in a house with the water heater in the basement; within a trailer it would be hard to get the elevation and slope of the pipes to what it needs to be to develop a worthwhile convection current. Not to mention, the convection method will be circulating hot water out of the heater continuously and uncontrolled. The cooler the weather, the more the heater burner runs, without turning on a single faucet.
I was thinking instead of having a small pump to recirculate the water; this would work well if you have, or would like to add radiant heating now or in the future. The pump could be activated manually when needed, perhaps a button by each hot spigot would be crude but cheap and effective.
If you wanted to get real fancy, you could have it run on a longer or shorter duty cycle, say 30- seconds on, 3-5 minutes off. Increase the percentage of on-time the colder it is.
I was thinking of ways to make the system perform better, to get a longer duration of hot water for showers. One way is to have the water not recirculate to the heater directly, but instead to a (let's just call it) Boost Tank. This is a small (3-8 gallon) tank that should be located near the existing water heater, preferably within the same insulated space. The water heater would draw the hottest water from this tank as its input, and when recirculating intermittently to faucets, the boost tank would become somewhat warmer than ambient temperature, enough to allow a substantially longer runtime than possible when cold hookup (or tank) water is going into the heater directly. Incoming cold water blends with prewarmed water in the boost tank before moving on to the heater. If you really wanted to put your engineer cap on, you could add a small solar loop to the boost tank.
In case you haven't guessed yet, I LOVE overengineering things!
It seems I love the mountains and deserts more than my friends do. I sure miss them!
1971 Streamline Imperial project "Silver Snausage", 1985 Coleman tent trailer, 1964 Little Dipper, 1975 Northwest "Proto Toyhauler", 2004 Harbor Freight folding, still seeking my Airstream.