Going to reply to a couple of replies at one here.
Originally Posted by KAS
Wouldn't it be easier to just plug in a fan (12V if you have no shore power) to circulate the air? You could direct it towards the ceiling to turn over the air better.
A fan in front of the heater is not needed, just buy a heater with a fan. If it blows then just turn on the ceiling fan for better heat circulation. Yes it would be but as someone else mentioned it won't heat the under belly at all. This would defeat part of what I would like to do.
Originally Posted by Bpenn
Is there a filter on the cold air return like in a house?
No, there is no filter but probably is not needed. The returned air goes right into the combustion chamber. I would think any, or at least most, dust drawn back to the furnace would burn up there. You could place a filter in front of the decorative grill but I wouldn't, you may restrict the air flow too much if you used a good filter. The grill will act as a leaf catching filter. (cheap filter)
I have been thinking about a booster fan, actually two of them. I would place one under the kitchen sink and just crack that door open an inch or so. That would supply more heat to the underbelly than it would normally receive. I have a decorative grill under my tv set. This is where I considered placing the second one. This would supply better air circulation in the area where the water pump is.
A better solution to the water pump might be using a heat tape on the pump and plumbing in that area, or a small electric thermostat controlled heater. Jus need a little heat to keep any water in that area from freezing. Nothing else in that area really needs any heat.