20 to 40 feet is pretty long to find in the normal camping stores. This is generally custom made up.
If you hunt around, you can find 5 foot and 8 foot ones, Coleman brand, Marshall Brass and Mr. Heater. http://www.marshallbrass.com/rv.htm
Marshall brass makes an "extend a flow". A hose and brass tee on your main tank for high pressure appliances.
Walmarts, Camping stores and even on line have them and they have the little green tank fittings (1” -20 thread) on them.
I have 2, 8 footers, use one on the gas stove and 1 on the gas grill run from my 5 lb LP tank.
A few things just in case you did not know and to save you a lot of money in case you did not. TT, LP gas lines are low pressure “inside” the camper. Low pressure meaning 11” of water column or slightly less then ½ psi. This is a safety pressure and is part of LP gas code for internal camper use.
If you have the standard Sunline setup of recent years, Sunline put a pipe on the side of the camper for a low pressure RVQ style BBQ. It just tees’ into the main gas line that is already regulated low pressure. It will run low pressure gas appliances of nay brand.
However a number of “outdoor” gas appliances are high pressure. Like a Weber grill, or a 2 burner LP stove etc. These are in the 5 to 7 psi range. Out doors they let you have more BTU heats and allow the higher gas pressure. These run off a LP gas bottle and are regulated down to the 5 to 7 psi range.
If you have a high pressure appliances and run it off low pressure, it will not do a whole lot. And if you have a low pressure device ruin on high pressure, you could get sky high flames.
They do make low pressure stoves like a Camp Chef cast iron burner stove and the RVQ, BBQ etc. So I do not know which you have and you might be OK. Or you might have a Sunbeam grill that runs on a little green gas bottle that will not work on the side of the camper gas pipe.
Another thing about gas hose. It is a needed accessory, however it also collects an oil that come out of it from the LP gas over time. The longer the hose, the more the oil. And eventually the oil will come out the ends with the gas pressure and clog up the down stream gas device some times. If you actually get a continuous 40 foot length and use it all the time even on short runs keep a heads up for that oil. It takes time to build up, may take 20 to 30 uses but it will come and the more rubber hose the more oil. Having the hose vertical with the ends open to the atmosphere will drain it over time. It might be a challenge to do 40 feet straight up but a couple coils and it will drain.
2 shorter ones or 1 short one, one long one etc might work out better pending you use.
Good luck and help this helps.
PS They also make low pressure hose and high pressure hose. It all depends on which side of the regulator you are on. Right off the main tank could be 150 psi hose. Down stream of the dual regulator could be low pressure hose, 5 psi range. Care full which one you are wanting. Using high pressure hose in a low pressure applications is OK but not the other way around.