Where the joint meets, is it subject to getting wet? If not, you don't need to caulk. You may be making a mistake if you DO caulk also. The shower/bathtub surround needs to have some flexibility. Not only for the slight twisting of the walls and such when on the road, but also because of temperature changes between sub zero in winter in Northern States and over 100 in Southern Florida in the Summer.
If you affix the surround and there is no "give" the screws will break out the surround, and will crack. The wall behind and the plastic surround will expand and contract at different rates, causing stress on the plastic and will cause it to crack, sometimes from top to bottom.
I had this happen on my last camper because the screw holes were made by simply screwing through the surround right into the panel behind it. Absolutely no give. One winter it got below 20 degrees, and the bathtub surround cracked in several places, originating from every screw hole.
My current camper has much larger holes, and the manufacturer used a plastic type snap in each hole, allowing for expansion and contraction. The only caulking is on the inside of the shower stall where the glass wall joins the surround. There's a bead of caulk there, and the glass wall and door is caulked at the bottom where it rests on the shower tray (basin).