You might look for a plate on the furnace to determine its model number and then to some Google searches for more info. Just searching for Hydroflame brings up too many results.
If it's a Hydroflame BRC-10, for example, here's an installation manual for that one.
It appears the BRC-10 is a gravity (convection) furnace and according to step 8 on that last page, a fan is an optional add-on.
(I couldn't read the instructions online but could barely make them out after printing them off)
Also-
If it happens to be an ARS-10XL, here's a Sunline Owner's Club forum thread where a guy got his working.
And for those of us just interested in vintage RV furnaces, here's a video of a '68 Hydroflame in operation.
Also- Interesting note: The BRC-10 installation manual says to connect the optional fan to DC power. However, in this PopUp Explorer thread about the vintage Suburban gravity heaters, we learn those have a fan powered by the heat of the furnace....
"On the models that have the little electric blower; the blower is powered by 3 thermocouple components that create voltage when they get hot, just like how a thermocouple works in other gas appliances where it's voltage is used to energize a magnetic winding and hold a gas safety valve open. They're wired in series and the hotter the burner box gets, the more voltage they make. The little blower fan spins according to how much voltage it's receiving. I believe this "technology" is used on some modern fireplace inserts. Pretty neat idea."
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Edited by Old Crow, 30 August 2020 - 03:45 PM.