Newbie here. Just sold our popup a while ago and found a good used Salem 25RLSS. We have it in the driveway checking everything out and for the life of me I cannot get the furnace to light on this thing. The fan comes on and you can hear the DSI try to ignite it but no go. I took the tanks in today and had both of them filled. Hooked them up performed the purge cycle then lit all three burners on the stove for a while then tried the furnace multiple times again with no luck. Then I turned on the fridge and operated it on gas and it appeared to be working. What am I missing here?