I know someone will come along with the same experience and have some input, so I'm anxious to hear from you. thanks in advance!
Here's the bulletin and reset procedure for the power sliding door(s).2000 XLE here with the same intermittent door problem, and here is what I have learned over the past 10 years:
1) My door will not operate when the engine is off if the battery is the slightest bit weak or discharged. It is a reliable indicator that the battery needs to be replaced or at least charged, far earlier than the inability to start the engine. Failure to operate also seems to be temperature dependent, when it is very hot or very cold - possibly related to the battery condition.
2) You do not need to cut cables - I'm sure you know (maybe not?) that the door is easily operable by hand if the power switch just left of the steering wheel is turned off. In many cases mine will operate by power if that switch is turned off and then back on after closing the door by hand. I don't know for sure but I think turning the power switch off and on resets the door ECU.
3) The inside door button on the passenger-side pillar seems to get stuck or fail to make contact sometimes. Poking it at different angles sometimes makes it work, other times the power door button on the dash will do the trick. I have seen posts where someone took the switch cover off and greased the plastic sliding parts with silicone, and another where they just sprayed lubricant in around the button. I haven't tried either of these, but once it started working after giving the surrounding pillar a good thump (I could hear a little *tink* as if something stuck was being released).
4) Keeping the contacts on the door and frame clean and free of tarnish may help.
5) I complained about this door early in the life of the van, and the dealer only applied extra grease to the tracks, which helped a bit but not much. I have read there is a TSB out for fixing the problem, but it might be pretty expensive.
Hope the above helps. On the mileage issue, I think you should be getting a bit better than 20 mpg unless traveling far above 78 mph. Mine gets about 22 in mostly-highway driving, and a solid 23 on the freeway using cruise control. Yours might be in need of "tune-up" maintenance, although I must admit never seeing much difference after such service.
BTW, the 2011 in my Username refers to the new Camry V6. Woohoo, we like it - getting 30 mpg in 80% highway driving so far (4000 miles).