I don't know your floor plan or where your water heater is on the unit ; SO ; you need to be inside the unit where water heater is on the outside .
Look for a panel , mounted with screws ; could even be the floor of a cupboard ; that needs to come off/out for access to the back of the heater and hopefully you'll find valves factory installed for by-pass .
Other wise you'll need a kit , to install your own valves .
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.