Is anyone using solar hot water heaters?

I have perused a number of forums and people seem to only be using PV panels on their roof to generate electricity and then using electricity to make hot water.  Seems like a waste to me.  I will be buying an 18' travel trailer soon and will put pv panels on top - should be reasonably easy to be self sufficient, given the small amount of electricity that I use.  I will use propane to heat and cook but it would be nice to not have to use electricity or propane to generate hot water. I think I will have an 8' by 3' area that I can use to install a hot water system.   Any thoughts?

PV panels are so cheap it is cheaper to use the excess electricity to heat water than build a dedicated water system. It also gives you lots of extra electricity for winter and cloudy days. You don’t have to have extra batteries, just panels and a load control so it only heats water when the batteries are full.

For both space and weight issues I would say no to it in travel trailers.  I am a firm believer in hot water heating for stationary homes.  But the weight is simply to much to be made mobile.  To be effective you probably need a minimum of 300 gallons.  Which is roughly 1 1/4 tons of water.  That is 3'x3'x4.5' for the water  plus roughly 1 foot of insulation so now you are looking at 4 x 4 x 5.5 = 88 cubic feet.  Add the blend valves, pumps, back up hot water heat and guessing you are solidly over 100 cubic feet.  Worse yet for efficiency reasons the ideal tank is taller and skinny down to cubic in nature.  Best for the trailer would be thin but big so you can put it under the floor.  Now you need more insulation.

I use a thermosyphon flat plate collector system on my house.
I have a gravity feed to the household tank in the roof of about 800L capacity.
It works fine for 4 people in summer.

We have a thermosyphon system to heat our hot water.  We have PV cells to generate electricity.  In my experience, people get confused between solar hot water and solar electricity.  Unless the hot water heater collector is raised from horizontal, the thermosyphon will not work.  There are some new technologies such as evacuated tubes that are heat transfer systems but from my understanding are too fragile to be installed on a vehicle.  These days, there are instantaneous hot water systems that are very efficient and way cheaper than solar and the difference will buy a lot of gas.  Solar panels on the roof will make an air gap which will keep the sun off in summer and provide a warm air gap in winter.

When I bought this house in 2021, it had a rooftop solar water heater which we still use. The tank is roughly 50 gallons (covered with insulation, so hard to be sure. The collector is about 6' x 6' inclined tubes under glass. It appears to be all black anodized aluminum, except possibly (hopefully) the tubes themselves. The anodizing peeled off, and it was pretty shiny. I removed the glass and spray painted it black again, then siliconed the glass back on. We are in Puerto Rico, so it gets plenty hot most days. If there is 100% overcast day, we are still good. The second day of overcast, the shower is warm, but not hot. The third day of overcast, the chill is gone, after that, we just wait for sun. I switched the house from city water to rainwater in 2021, so I guess the more acidic water in the pipes will eventually eat through something on the roof and I will have to replace. But replace I will. There are all-stainless systems for sale now.

We have solar electric too, but it is not sufficient for water heating or electric stove or electric dryer. We use propane in the stove and hang clothes to dry. There are just two of us and our PV array is 2kW. Our inverter is 4kW and our battery bank is 9kWh. Our water is all pumped twice - once to a roof tank and again to pressurize the house plumbing (I did not set it up that way, just how it came mostly - the roof tank used to fill from city pressure, but is not high enough to service all the skinny 1/2 pipes. Some day, I will add a second independent solar array for charging an EV and running an induction stove - maybe even an air conditioner if things keep going the way they are. But we are at a good elevation here and it isn't hot like down on the beaches.

here in Brazil we see a lot of passive solar rooftop things (keeping in mind it's only used for showers, not for washing dishes, laundry, etc). Usually a small barrel, maybe 13L, with piping that goes under a dark panel.
We used to be very involved with the custom car crowd and got to see a lot of interesting travel vans, mostly VW vans, and a surprising number of them have two large-bore PVC pipes painted black and mounted under and alongside the roof racks for rudimentary hot water showers. It's not a huge volume of water but you don't have too many people living in a single VW bus (hopefully) and also it's not heavy enough to cause structural issues.
For a larger trailer, I'd be wondering about the integrity of your roof if you have anything big.

Plugging into Paul’s post above, I have a very basic system for my barn. As is usual for me, I have a dual system. I use a 30 gallon black tank to heat water in the summer.   In colder weather I have an on demand lp heater. Both see minimal use, but they are good for a fast shower, washing hands, or scrubbing a stall.

This is a timely sort of thread 'bump' for me. My DH is catching up on Maximus Ironthumper's  youtube videos. He's an interesting man living off gird in the UK with some discussion about wet heat solar thermal versus PV solar thermal. Now I'd always leant towards a water loop system, but the complexity of fitting that sort of system in our house has always put me off doing it at all. To get a nice thermosyphoning system that links into our existing stove/hotwater tank set up turns out to be complex and inefficient. Our hot water is at the North end of the house - our roof faces East and West, we would need a new hot water cylinder with another coil, the hot water cylinder is too high in relation to the roof, and so on. The key advantage of PV with electric load into water heating is that that geometry constraint is broken. Electricity doesn't care about gravity! Wires can go through walls where they are least likely to cause leaks, rather than being constrained by geometry to a single location. My thinking is turned on it's head and I've got a whole new rabbit hole to go down.