Project planning rad floors & radiator style towel racks

Ranching

So I've been reading a lot on here and way too many other sites. To educate myself better on hydronic heating, but I have a few questions that I just can't seem to find an answer to and would like maybe some possible recommendations.

So I'm making plans to build next year a camper from scratch. I've done plenty of home building and remodeling and have been toying with ideas for basically the ultimate luxury 22-foot camper for a while. It will be well insulated vs most on the market and I'm aiming to make it as energy-efficient as possible for off-grid camping spots. (I ranch so it would be nice to put it in the pasture during winter calving season)

So I'm going to be putting in it a prestige 9.0 gpm propane combi boiler. (Yes I know this will be overkill for what I'm doing, but I can't find anything smaller either). With it, I plan on putting radiant floor heat using Quik trak, transfer plates, and 5/16 pex at 7" spacing. The footprint though that it can be run is quite small due to cabinets and such. I should only have about 49.7 sq ft to be able to run the pex. I know if run it right this can still get me just short of 1500 btu. The BTU requirement will be roughly 3864 due to the extremely low temps it may see. (Yes trying to plan for that to be in the build to avoid the possibility of freezing)

The camper's total sq ft on the blueprint to be heated will be 147 sq ft. I'm estimating I will still need to come up with another 2372 btu. So I figured I would get two Hudson reed towel racks as they are steel and are mounted at 4 points into the wall (This is important to me as being a camper not sure regular rads would hold up well). One would go in the main room the Arch - White Hydronic Heated Towel Warmer - 29" x 19.75" which is rated at 2567 btu at delta T60. The other will be in the bathroom the Ive - White Hydronic Flat Towel Warmer - 23 5/8" x 15 3/4" which is rated at 748 btu at delta T60. I figured I would run the floor and the rads on separate loops/zones. Using the floors to primarily just help hold temp. Also the bathroom one is sized to fully cover the bathroom sq ft and the other the main room as if they are separate.

Now that the explanation for my thought process is finished time for the questions. I plan on running them all at around 130F. What size pex would be recommended to run the rad towel warmers? Can I use the same size as the floors or will it need to be a separate size? And any idea what psi or gpm the towel racks may need to see? Also, I'm a bit lost when it comes to determining what is needed to provide adequate heat to the warmers to achieve the rated btu and delta T.

(Thankfully I have plenty of time to figure all this out of course as I'm in the blueprinting/planning/budgeting stage. I have plenty of experience with hydraulic systems and mechanics as well as heating and plumbing, but combining it all in this way is a new beast to me. So I figure expert advice or even pointing me in the direction of reading materials to learn more would be great!)

hot_rod

I built a 20’ tiny home a few years back.
I-used the Roth panels with 3/8 tube 6” on center. 

130 seems high for a floor system?

I used a kick space heater for back up assist, although I never heard it kick in

A small electric boiler works nicely for low loads

There are some small RV boilers that do dhw and heating

They run on diesel or gasoline. I suspect there is an LP version. Crazy $$ so I went electric 

https://www.heatso.com/espar-d5e-rv-heater-kit-12v-5kw?sku=S3-RV-IT&gclid=EAIaIQobChMImujhrb-hgQMVkxp9Ch22VwMOEAQYAyABEgKEZPD_BwE

Ranching

This will be used in the winters in Minnesota, Maine, and Montana so overbuilt is the only option when it may see -30 out. From my understanding staying between 125-130 is a safe bet, but I wouldn't want to go over that range. I don't plan on running it that high all the time just those days in January through feb that it wants to drop at night.

And yeah I've looked into those as well but the entire thing will also be running on 120v with inverter full batter bank for about 24 hours worth of power paired with a larger generator to recharge them. So energy efficient appliances and all that stuff. Primarily just to extend how long I can be here before having to run to town. (I'd like to make it a once-a-week to every 2 weeks situation instead of every 3 or so days). With the weather extremes and my personal needs in it I'd rather just go full hydronic. The rest of the camper will be modeled after a 60s Shasta but built similarly to a net zero house with some modifications to account for vibrational resonance on the structure from road travel.

Ranching

Also for anyone wondering I'm aware I'm doing this an unneededly harder way to achieve what I want this project to do. But part of this is to learn and I want to figure out if there's a better way we can do campers and mobile living. And I'd rather be my own guinea pig for this as I have a degree in engineering, but I'm a traveling cowboy (Strange combination I know). So this Project is also for my own amusement as well. So if anyone wants to add to this even just for a thought experiment and would like to see said thought experiment come to life. This is essentially what this project is about.