Hot water heater makes loud boom

Thanks all you wonderful people, will give it all a look, I noticed during the night that it makes the boom again as it reheats up, so don't feel safe with this at all. We are parked on a perm site we now own, so this camper is for long term. Hope we can fix it.

If Old Biscuit's advice doesn't do the trick (it almost always does), check the gas valve solenoids. My heater had a similar problem, it would fail to light and stay lit, but in attempting to light, there would be quite a boom and at night, you could see the flame on the outside of the heater door. The solenoid would let in enough gas that it would eventually ignite, but not just burn evenly and stay lit. Checked each of the solenoids and found that one of them usually didn't open fully, so full flow was not established. New gas valve including solenoids was the answer.

I was given my 70's model atwood. Fired up o.k., but lots of black smoke kept coming out of the exhaust... turned out to be a dead colony of lady bugs.

Delayed ignition check for the causes

Thanks for the help, will def. do this. Because we know that it has been sitting at dealer a few months before it was sold.

moved from technology corner

BOOM is due to dirty obstructed chamber...spider webs/dirt dabbers and a disrupted air/fuel mixture

Clean combustion chamber and burner tube.

Remove burner tube/spark electrode and then run a small bottle brush thru burner tube

Then run a wire thru combustion chamber until it comes out exhaust....tie a rag to wire then pull it back thru chamber.

When reassembling check that burner tube is in a straight line in regards to main gas orifice (gas should flow straight done tube...not bounce off walls of tube)
Flame spreader on end of burner tube should be 'squared' with end of tube.

And main flame needs to engulf tip of spark electrode