With 2 disposable propane bottles of 22 kBTU (thousand BTU also sometimes MBU) each, an 18 kBTU/hour heater should run for 2.4 hours. Still going strong after 3 hours made for a puzzle. For the first clue there was the manual noting that the high heat setting with the 18 kBTU/hour rating required 2 bottles. A second clue is seen in taking a look at the propane bottles. (see Amazon for these heaters and accessories)
note the frost on the bottle! The bottle gets cold enough for that frost because the liquid propane evaporates to provide the vapor for the heater and evaporation needs heat. That much frost indicates that the rate of evaporation was quite high for the size of the bottle. That implies that the heater was running a bit starved so its propane use was a bit less than its full specification. That is why it ran for longer than a simple calculation of fuel stored versus fuel use rate would indicate.
One way around this would be to use a larger propane tank like the one on your RV. That has a lot larger surface area for the liquid propane which means a higher evaporation rate and larger vapor flow possible out of the tank. For that, the issue is pipe size and fittings. The Engineering Toolbox has a page on propane gas pipe sizing that can provide a starting point. Note that 3/8 copper tubing is common in RV’s for appliances. The water heater may run at 15 kBTU/hr, a stovetop burner at 5 kBTU/hr, and a furnace at up to 30 kBTU/hr. The propane in your system is low pressure at 11 inches water column or about 0.4 PSI above ambient (i.e. gauge pressure). The heater in the picture has a quick connect fitting for attachment to low pressure propane sources on the other side.
Mr. Heater advertises a kit for connecting the heater to a low pressure RV system. It has a 3/8 flare one end, a 12′ length of 1/4 inch flexible hose, and a quick connect to attach to the heater (their two larger models have a connector for this this built in) on the other end. The 3/8 is the common fitting on RV systems. The 1/4 hose should be able to deliver sufficient gas for the high heat setting but the quick connect fitting says its rating is 16 kBTU/hr – a bit short. Note also that there are a variety of quick connect fittings for propane and you need to be very careful to use the right ones for a proper gas tight fit. The Mr. Heater fittings have automatic shutoff on disconnect for both ends and not all such fittings do.
You can also get fittings to connect a propane tank to the high pressure fitting on the heater where the bottles usually go. The problem to watch out for when using this method is the aromatic oil they put in propane so you can smell leaks, even small ones. Mr. Heater has a filter to use to keep this from gumming up the heater.
It is also interesting that the Mr. Heater auxiliary tank kits have warnings that they are for outdoor use only even though the RV kit is advertised for inside the RV heater connections. There are also California Cancer warnings. And there are propane warnings. And there are combustion inside warnings. So many warnings tends to flood the zone and hide what is really important. If you use one of these heaters you need to make sure that there are no leaks. Mr. Heater suggests the soapy solution bubble method on all fittings. Your RV should also have functioning RV rated detectors for combustible gas and carbon monoxide. Be aware of the signs and symptoms of these gases and know what to do if you have any suspicion that you’ve got them or if any detector sounds an alarm. Take care. Stay warm safe.