Hi Profdant139,
I've never seen a "mini" catalytic heater. It would need to be fairly sophisticated to lite and be thermostatically controlled.
Since you are concerned about tank freezing here is what I would do.
Add an extra connection to the furnace plenum that connects to the "newly enclosed" tanks are. The valves need to be in this heated space, too.
Then add a mechanical thermostat with a 12 volt in the duct fan (at the "box" you have created). Set the temperature for about 5 c (40 f). It will cycle and draw warmed cabin air into the tanks area. For best results, have a cold air return tube on the other end of the "box".
My solution, since I have a large battery bank, was to use a dual window fan to replace the cold air return on the furnace. Fortunately, for me, this pressurized the duct work and keeps the fresh water side of the RV above freezing. The fans draw a scant 27 watts.
For the waste tanks (because some one failed to connect the tanks area to the furnace plenum) I use a 250 watt heater (Walmart $9.99) and a mechanical thermostat. I've tested this down to -27 c (-16 f).
Another possible solution is a 12 volt heating blanket fastened to the tanks (and then enclosed). These draw about 7 amps (@ 12v dc). Again, add a mechanical thermostat.
Energy use is not too high, because all we want is for the tanks to not freeze.
