Ah, now that's a whole different scenario now.
If the caravan is not ever going to be towed on a road again then you don't have to worry too much about adding weight to it, so tiling is clearly an option. But you will need to construct a very sturdy enclosure to ensure there is โno flexing that could cause any tiles to crack. Don't forget that as well as the weight of the tiles there's the weight of the shower occupant to consider! Tiling that small space will involve a lot of tile cutting.
How bad is the crack? In the past I had a shower tray in a motorhone crack very near to the drain point (the usual spot) . Rather than rip the entire shower unit out I drilled into the void between the shower tray and the wall/floor (from the garage of the MH) and injected a load of builders foam! It filled the void, supported the bottom of the shower tray and best of all, its 100% waterproof so nothing leaked out of the tray! The repair lasted the further 3 years I owned the the MH.