How can I connect a portable propane bottle to a tankless hot water heater?
One major problem is that most propane regulators are intended for small appliances, mostly less than 75,000 BTUs/hr typical of an outdoor grill.  The Rinnai V65EP can use up to 150,000, so a regulator designed for a little more than that would be the desired fit.

Shopping around, I notice many don't say what their maximum flow rate is, and of those which do, it is hard to find a "low pressure" (corresponding to less than 13.5" water column) in that range.

Here are a few I could find after several minutes searching:


http://www.ebay.com/itm/Regulator-Stainless-Steel-Hose-Type-1-LP-Gas-Grills-Parts-21-BBQ-Low-Pressure-/261136984141#vi-content
http://www.ebay.com/itm/Propane-Regulator-POL-LP-Gas-Low-Pressure-grill-BBQ-smoker-stove-Parts-/251219429378#vi-content


Lowes has a two stage regulator capable of up to 200,000 btus.  It is inexpensive at $40.