I have installed a few radiant floor heating systems in RV's, including my own bus conversion. These work well if designed and installed properly. Most motor homes and bus conversions have very high heat loss and will require more heat than a radiant floor heating system will furnish. Adding a fan coil to the front loop of radiant will normally add the extra heat needed. Many people also leave the central system that the factory furnished with the coach.
Ceiling height may determine how much insulation and buildup you can use. Laminated hardwood floors are best for radiant floors. Floor tiles that glue down will not work on radiant floors. Quality roll out linoleum type material works best. Flooring products that require gluing are not good. Never put heat under a refrigerator! Do not use more than 100' of 3/8" Pex in each loop. Use Pex with NSF label, approved for domestic water use and heating, Locate manifold centrally. Angle hole to bring tubing up through the floor. Protect tubing when going through holes in bulkheads. RV's normally use over 50 BTU's per Sq. Ft. of area, 50,000 Btu's will normally heat RV and water. Mark on the floor where tubes are to prevent damage.
Several things about antifreeze that need to be noted, first antifreeze reduces the thermal transfer. It thickens the fluid and slows the flow down plus it causes a thermal layer next to the metal. And it does cause damage if it is not changed regularly. On the other hand it is very necessary for other reasons, for freeze protection and for corrosion protection if there is steel in the system. Using a 30 percent solution is a good comprise.
3/8" Pex Tubing A or B not Al-Pex
1/2" to 1" high density insulation, (Pink or Blue Board) not Styrofoam!