If other gas appliances won't work properly either, LP could be low or one's regulator could be slowly going. If new LP tank doesn't work, I'd investigate the LP Regulator next. Then, give your furnace an in-depth cleaning.
Every 4-5 years, I physically remove my TT's heater and clean it out. I shake them upside down and use a high powered air host to blow them out (while wearing glasses). Just last week, I 100% removed my 2006 Jayco's heater and cleaned it out. Man, you should see all the dust, grime and mouse droppings inside that unit. While removing, I inspect all piping, ducts and where needed, make "better then factory" repairs. If your RV is older then 5 years, I'd recommend a 100% cleaning (removal and air blowing) as well. I was surprised with all the "junk" inside our TT's furnace.
BTW: Many thanks to Rustic Eagle for posting " http://www.jaycoowners.com/showthrea...highlight=fire
". His thread reminded me to pull my TT's furnace completely out (since its now 5 years old) and give it an in-depth cleaning. Did I mention that I was amazed on how much lint, dust and mouse droppings inside our TT's furnace as well? Glad I disconnected its brass hose, removed it and gave it an in-depth proper air blowing cleaning. Time well spent. Thank you for the reminder Rustic.
Hope this lesson learned helps many....