Shower stall caulking.

The shower pan should have a 2" lip behind the walls, and I agree with previous comments -- this gap shouldn't be caulked. When I installed a similar style shower at our home, the instructions were pretty clear about leaving that gap between the pan and the wall panels open. Depending on outside temps, you might wind up with condensation which will run down the back of the wall panels, and that needs a way to escape. Caulking would trap it.

More than likely Winnebago used a Silicone caulk for this. You will never get new stuff to stick without removing all of the old stuff. And I mean ever trace. Probably more work than it is worth.

I have read, some years ago, that this joint, between the shower pan and the walls, should not be caulked, because caulking this junction will close off the space behind the shower enclosure so it cannot breathe, and moisture will then allow mold.
Considering how loosely my rig is put together, this seems unlikely.
Mine came from the factory caulked.

"An ounce of caulk is worth a pound of cure"....well, in my book anyway. Probably not necessary but I re-caulk this seam about every 2-3 years just to be safe. Cheaper than having to remove shower for moisture/mold issue. I use Duralink 50 which stays 50% flexible for life (useful for RV that's constant vibrating/moving). I use this stuff for windows/vents/shower/exterior lights etc. and have had very good results.

As long as the walls overlap the pan on the interior side of the pan, it's probably not necessary, except in the even of an overflow in the shower pan itself, but that would be another issue entirely. That caulking would then be the least of your worries.

While water should not get over the pan lip, the caulk keeps water from sitting between the inside of the wall panel and the lip of the pan. That would not be accessible to clean and mold could form.