Great thread. Malconium, I'll be excited to hear how your project turns out.
I installed radiant heat in my 100 year old house a few years ago. We are in our third heating season with the system. What we're doing is similar to what you describe in that we have PEX tubing retrofitted underfloor.
I tend to obsess over BTUs and amps and stuff of that nature as some of my other posts will attest.
The heat loss calculators are worthless for anything other than conventional post-1980 residential construction in an area protected from the wind. Don't rely on them for anything else or you'll make expensive mistakes.
The only way to find out how much heat it takes to heat an Airstream trailer in particular conditions is to actually do it and measure the heat input. The amount of heat required is roughly the same regardless of source with the exception of forced air which has a loss of typically 10% due to the additional air leakage. Airstream is currently putting 34,000 BTU/h (input) furnaces that are 85% efficient into their 31' trailers. I don't see anyone complaining that these are overkill or, conversely, that they are too small. What we hear usually is that they are good down to aroudn 15 degrees in the wind at which point they'll run all the time. 34,000 BTU/h * 90% forced air efficiency * 85% furnace efficiency = 26,000 BTU/h.
Another data point is that the people who heat only the bedroom using an electric heater during relatively mild weather seem to be fairly happy with the 1500
watt "big heat" portable heaters. 1500
* 3.143 = 4,700 BTU/h. They're heating maybe a third of the trailer, so 4700 * 3 = 14,000/h BTU, in mild conditions (usually above freezing). So I think for planning purposes you aren't thinking in terms of enough heat.
You can measure system runtime ratios to scale the results. So if you sit in a trailer on a cold day and have the 34,000 BTU furnace running 40 minutes out of the hour, say, well then you can multiply the figures by 40/60 or 2/3 and that's how much heat the trailer needs.
I run my radiant heat system at home at 144 degrees. It uses 1/2" pex with the aluminum heat plates stapled up around it to the floor. I have water-flow measuring devices on each zone and can measure the temperatures on the inlet and outlet of each zone with an infra-red thermometer. The floor is a 3/4" solid wood subfloor with 3/4" wooden plank flooring above it and the usual mixture of area rugs and furniture you would expect. I have insulation to about R-20 below the pex over an unheated basement. What I have found is that the largest zone, which heats 300 s.f. of floor space, will not transfer more than 10,000 BTU/h. At this point the floor is quite warm to the touch although not unpleasantly so. The floor area on your airstream is somewhat less than that so you have to realize that you're just not going to get more than say 9,000 BTU from radiant floor heat no matter how well you run it, unless you get the floor too hot for it to be comfortable to walk on, with the attendant problems this poses for floor coverings, electronics, warpage, drying, etc.
Regarding flow rates, the rule of thumb is that you want to design the loops so that, with 1/2" pex, you get 1 GPM in 300 feet of PEX with a 10 degree drop which then heats 10,000 BTU/h. If you have lower BTU requirements you can still run 1 GPM and you'll get less than a 10 degree drop which is fine. It is possible to run up to 2 GPM through the PEX in which case you will get a 5 degree drop at 10,000 BTU or you can set up a longer loop or one with some baseboards and get 20,000 BTU at a 10 degree drop.
The pumps used in radiant heating can be throttled with a flow reducer or an ordinary valve, so that you get the flow rate you want. It shouldn't hurt the pump. If you have some sort of goofy pump that doesn't like to be throttled well then just plumb in a bypass loop around the pump with a valve you can adjust so that half the flow rate goes in a circle around the pump.
Much over 2 GPM and you risk abrasive damage to the piping system in 1/2". It's like sandblasting with whatever sediment made it past your filter and whatever corrosion products have become suspended in the water over time.
In general you do want to keep the domestic water and the radiant heat water separate, mainly because the dissolved oxygen in domestic water will tend to cause boiler corrosion. I run a glycol antifreeze at home having lived through the pipes freezing in a hydronic system in the house where I grew up. Not pretty. In a modern system the PEX will survive but any metallic components e.g. fittings in the system are likely to burst especially with repeated freeze-thaw cycles.
In our house we have had to add baseboard units in order to get enough heat. It was done after the fact and ended up being a big hassle and quite costly. Best to design them in if you think you'll need them and from the math I think you'll need them.
Before there was PEX people used 1/2" soft copper. Expensive and prone to leakage where solder joints are used, and generally many solder joints are required because you can't lace copper through a bunch of turns the way you can pex. In practice you won't get more heat output from copper than PEX, because the heat transfer through the wall of the PEX isn't going to limit the amount of heat you get, the amount of heat transfer through the floor is what will set the limit.
Our house has heat that actually works and every room is comfortable in wind and weather.That is rare for buildings in Minnesota because almost all of them are heated with forced air and there are cold spots and hot spots that move around depending on wind direction and whether the sun is shining. Aside from some pipe creaking at times due to expansion our system is silent.
Radiant systems don't inherently require much longer to heat up than forced air with the exception of system where the radiant coils are deep in a concrete floor. However in practice most forced air systems installed in houses are oversized while radiant systems are not and this makes for the situation where it takes several hours longer to heat up a cold house. A 60,000 BTU furnace costs about the same as an 80,000 BTU furnace and what happens is that the heating contractors use the same size furnace in most of the installs that they do, nobody calls to complain if it's too big and they get their chain pulled if it's too small. In practice the oversized furnace does pose a number of problems but that's outside the scope of our discussion here. I don't believe the same logic applies to Airstreams and would guess that the 34,000 BTU/h units being installed are a pretty good fit.