First, take Tango's advice to heart. That being said, I have a Ecotemp on demand water heater built into my bus interior. I built a steel, enclosed case to house the heater and both water pumps. The enclosure is sealed to the interior and gets it's air through a filter screen in the floor. The exhaust is through a screened vent installed in place of a window in the rear wall of the bus. These heaters produce a LOT of heat and that heat must be able to freely escape. I don't believe that would be possible with the heater mounted below the floor. Also, I lined the enclosure with woven heat shield--the stuff used to wrap exhaust headers on hot rods.
I do have access holes to allow for adjusting the heater and replacing it's batteries. I can also access the pumps. The holes are covered with steel plates which are secured with many screws as they need only be opened periodically.
As you know, these heaters require both a constant flow of water and a minimum water pressure to light off. Ecotemp now offers a pump kit with a pump with adequate pressure and flow to get the job done. It is what I have installed on mine. I use a water recirculating pump to keep the heater from rapidly cycling on and off. It seems that there is a bit of fuel still in the unit when the igniter shuts off which isn't a problem unless the gas and igniter turn on again before the bit of unburned fuel has disapated. I discovered that if the on off cycle didn't allow for the disipation of the unburned fuel, there would sometimes be an explosive relight--something the proper pump and water recirculating circuit cured. I moved the igniter switch from the heater body to a convenient location in the bus as this allows me to turn the heater on and off more easily and saves on battery use and the associated headaches. Jack
In this pic you can see the heater enclosure on the back wall. The access holes are uncovered. Also, note the slanted top of the cover. This helps direct the exhaust out the top vent and keeps the unwary from using the top of the cover as a shelf.