Can't be positive , but the coach may have come with manual lock bars from the factory that a previous owner lost .
One fifth wheel I had was supplied with a 1 1/2" dia. metal tube with rubber cap ends to jam between the RV wall and the slide flange .
You could look at a building supply for this type of adjustable support , or fabricate a bar out of wooden doweling cut to the correct length and capped with a rubber " foot "
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.