You decide to go with a version of events that’s technically true. You can fill in more details if it becomes necessary later.
“I need to get back to my camp. I went out and got caught in a sandstorm, got badly lost. I was thinking your wagon would be a big help in getting back, if you were willing to lend it.”
They pause for a bit.
“What’s your camp doing in this desert?” they ask, a bit suspicious.
“We’re researchers. We’re studying the desert creatures, and trying to better our understanding of desert fever in hopes of devising new treatments.”
“Desert fever, you say.”
They glance behind you.
“And who are those folk over there? They seem a bit heavily armored for researchers.”
You were going to get to that later in your explanation, but it seems your hand has been forced.
“Funny thing,” you say. “They say they’re from the swamp, of all places. I’d think they were mad, but I’ve seen odd creatures popping up here, so maybe there’s something to it. Of course, it’d be easier to ask them questions if I had the rest of my crew to back me up.”
“I’m afraid I can’t really take on passengers,” they say. “One grebling is one thing, I could make an exception for the right price. But throw in five humans? No thanks. Way too much risk. You might break something.”
You’re getting the distinct feeling this human is hiding something. How do you handle this?
Ask in a roundabout and or casual way if they’re carrying delicate wares? Add that, being a researcher, you know how to conduct yourself around sensitive equipment and are Very Unlikely to break anything.
Tell them your research expedition has been generously funded and you could compensate them for their troubles. See how they react to the prospect of wealth that’s up for grabs.