Hotwater Heater

The burner tube sounds like it is plugged. Bugs or dirt maybe.

Needs basic service like cleaning chimney and adjusting flame.

Ok I'll pull the manual for it and run through a cleaning in the morning

Bug nest in the burner tube, most likely. Clean it out and you should be good to go. Resist the urge to poke anything into the gas metering jet. It is easily damaged.

Had that on my heater. After cleaning it still happened. Finally found a wisp of a spider web in the tube.

Ok I managed to find time and got the tube out, a quick look and there was just 1 little wisper of dust.
Cleaned it out and used a soft brush on the fuel tip.
Fired up with a nice steady blue flame.

How long does it take for the water to warm?

I believe I found the answer >>

"While they can vary in size, the most common size of water heater is 40 gallons, which is what we'll be using here when talking about how fast they work. The bigger the heater, the longer they take to heat up the water. The average gas heater takes between 30 and 40 minutes to fully heat up the water in its tank"

Typical RV water heater size tanks are 6 gallon, 10 gallon, and bigger for larger coaches. My standing pilot 6 gallon water heater takes about 10 minutes to fully heat the water.

Yes this 1 took about 15 minutes.
Works good now!
Thanks everyone for your advice :)