I've checked the 1-wire presence with a different PIC and different 1-wire device, and it seems to be working ok.

I noticed the point that your program jumps to doesn't give any indication that it jumped there.
Maybe you could try something like this for testing purposes.

Code:
Sensor:
    OWOUT OWnet,1, [skiprom,WriteScratchpad,$00,$00],Senout
    OWOUT OWnet,1, [skiprom,ConvertT]    :pause 10
    OWOUT OWnet,1, [skipROM,ConvertV] :pause 5   
    OWOUT OWnet,1, [skipROM,RecallMemory, $00] ; Skip ROM search, Recall Memory
    OWOUT OWnet,1, [skipROM,ReadScratchpad, $00]   ; Skip ROM search, Read 
    OWIN    OWnet,2, [STR DSbuffer\9]' Read 9 bytes
    Serout2 TX_485, 84,[" 1-Wire is present on main"]
    GOSUB GetT_V
RETURN

Senout:
    Serout2 TX_485, 84,[" 1-Wire presence NOT DETECTED!"]
RETURN