Have you looked , under the floor in the same area?
My last 5er, the low point drains were , a line through the floor, that was capped off , rather than a valve above floor level.
EDIT: On re-reading your post, I'd say open the valves you've found, with the water pump turned off and see what comes out. You could have hot water in the system first, to identify the hot line by , the temp of what drains out.
BTW: Welcome to iRV2
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.