After reading this thread and doing some experiments, I came up with a solution that works for me. I have been having problems with the water heater going into retry mode and finally quitting. The clue to open the access door was extremely helpful. It was very clear that the unit was being starved for air. I also camp at fairly high altitudes (about 8500 ft.) and the problems were much worse there.
Basically, I combined what I read here into one solution. For high altitude use, I added a fan on a switch so that I can use or not use it. Until I get back up to altitude, I won't really know how effective it is.
I picked up a 4" louver (they call it a 4" Mill Mini Louver) with a built in screen at True Value Hardware store for the princely sum of $4.
With a 4 " hole cutter, I cut the two holes for the louvers. With tin snips, I opened the back of the louver to accept a 4" computer fan. The fan only pulls 0.3 amps so nothing should get overloaded. I pulled 12 volts from the switched (left) side of the thermostat and ran it to my switch. The fan will only run when my switch is ON and the water heater flame is on.
I used the fan mounting holes as a template for drilling the mounting holes.
So far, it works great at home (about 1300 ft.) without using he fan. With any luck at all, I won't need the fan but wanted to try a comprehensive solution.
The swooshing' sound that went away when the door was opened is gone and the flame is much more consistent.
Closed view
http://www.irv2.com/photopost/data/5...035_Small_.JPG
Open view
Thanks to all who contributed, it was most helpful.