I don't own one, but figure it would take a lot to get the fumes to build up that much AND get access to the pilot light.
Chances are pretty good that even if you got a fair amount of fumes in there, that you would stay below the lower explosive limit. With that said, can you empty the tanks and put some cans in the truck bed safely? Empty tanks would probably not carry enough to get to the LEL in a trailer that size.
I work in the environmental field, and have had to deal with gasoline releases from gas stations that sometimes (very rarely) got bad enough to get vapors into residential basements. In those cases, it was very rare that the fumes would ever get to a point of being flammable/explosive.
If you do this regularly, and would like some more comfort level, there are sensors that can be installed with automatic venting fans that could be installed. One that we installed was fairly cheap and was capable of venting a 1,200 sq. ft. house in a few minutes. These are meant for industrial applications, and I think are available through Grainger and/or LabSafety Supply.
There is also a cheap product you could put on your vent stack that is meant for RVs, called the Lil Stanker vent fan, which would continuously pull air through your waste tank and out the roof. In theory, that would keep some fresh air circulating into the trailer and help keep the fume concentrations down. That fan is under $50.