nicole,
Thus far, this does not sound like a gas leak because even a tiny amount of gas makes a very noticeable odor - even standing outside, next to the van. I hesitate to ask the next question, as you seem to have above average automotive skills but here goes: Since you recently acquired the van and may not be aware of all it's nuances, could it be possible that what you are seeing is condensation from the air conditioner? This would present as a leak of clear fluid on the passenger side of the van near the transmission area. Depending on humidity/temperature it may be evident some days and not others. Just a thought.