I have fitted several of those, mostly on Iveco Daily work vans but some Ducatos, though in fairness not MHs as they tend to have more paneling etc.
Ideally if you can gain some access to the lower end of the actuating rod to keep it roughly in place it will be easier, however it should be possible with a little care to undo the bolts on your catch and unhook the rod then whilst keeping rod in place use your third hand to fit the new one in place.
You may find some long nose pliers can hold rod up whilst fitting new catch.
I have mentioned this before for other MH users on Forum if you can find the thread.
By the way any adjustment is done by the locating pin /bolt that the catch engages with as by slackening slightly it is in an eccentric hole.
Always assuming rest of door on sliders is correctly aligned and regularly lubricated with proper oil or grease not thin WD40 which lasts 5 seconds on that job.