I remember seeing another busnut's design where he routed channels for PEX tubing into plywood for radiant heating.
While I won't be using it, I came across a product that has channels with a preformed aluminum skin for better heat distribution.
(https://www.warmboard.com/sites/default/files/Warmboard-S_03.jpg)
(https://www.warmboard.com/sites/default/files/styles/gallery-large/public/26Warmboard-S_BluHomes_Factory_CA_0.jpg?itok=NFs-_lg1)
You have to custom order from the manufacture and it normally comes in 4 X 8 foot sheets.
However for remodels they have 2 X 4 foot sheets, that you could also install on the walls.
Seems like it might be a nice system if you are converting a bus from scratch and you need to replace the plywood floor anyway.
More info:
https://www.warmboard.com/warmboard-s (https://www.warmboard.com/warmboard-s)
Was closing down my browser tabs and found some more links that may be interesting.
https://s3.amazonaws.com/finehomebuilding.s3.tauntoncloud.com/app/uploads/2016/08/18055146/Warmboard-S_profile-700x490.jpg (https://s3.amazonaws.com/finehomebuilding.s3.tauntoncloud.com/app/uploads/2016/08/18055146/Warmboard-S_profile-700x490.jpg)
https://ecowarmradiantheat.com/ecowarm-specifcations/ (https://ecowarmradiantheat.com/ecowarm-specifcations/)
https://www.blueridgecompany.com/radiant/hydronic/527/rht-floor-panel-system (https://www.blueridgecompany.com/radiant/hydronic/527/rht-floor-panel-system)