Barbeque yes. Firepit no.
A big barbeque will suck fuel at the rate of 20,000 to 30,000 BTU, but a fire pit inhales fuel at 58,000 BTU.
58,000 BTU is about ALL a propane tank can deliver...as witnessed by the fact that the tanks tend to ice up from the massive fuel draw. https://www.propanetankstore.com/blo...-propane-tank/
My firepit tank often freezes itself to the ground, and when I go to move it or replace it, a big chunk of dirt is frozen to the tank.
A free-standing 20# propane tank has one job...feed the appliance that's connected to it. In this case, the fire pit. The regulator and the tank deliver all they possibly can to feed the firepit while it's running on high...which is usually most of the time.
But if you connect to your RV's propane plumbing, your firepit can starve the furnace, stove, hot water heater and fridge of the propane they need to keep the rig running.
The barbeque will play nicely with the other RV appliances, and it's usually on for only a short period of time, but the firepit is a glutton for fuel, and everything will suffer as a result...and your propane tank may freeze up entirely from too much draw...and the flow will reduce or stop entirely.
One more reason to NOT do this. A 20# tank of propane contains about 4.5 gallons of propane. Propane contains 91,500 BTU per gallon. So a tank typically has 411,750 BTU. At 58,000 BTU, you get 7 hours of burn time on the fire pit. If you have two tanks on the tongue, you can safely go a couple days "sharing", but if you are out just 4 nights and you run the firepit 3.5 hours per evening...not that much...you've drained both tanks.
So, you need to carry spare propane anyway...if you run a firepit...so dedicate a tank (or several) to the firepit and leave well enough alone with the tanks on the tongue.
I've been running a propane firepit for about 5 years now. I calculate 1 tank for each two days I'm camping. When lakeside and we're doing lots of swimming and other watersports, I calculate more burn time (to warm up after a swim) so it's 1.5 days per tank.
Up against purchased firewood (required in most places), propane is a bargain, and it's far safer and cleaner burning than a wood fire. Just don't expect your onboard propane system to handle more than it's designed to.