First, did the dealer check for leaks?
Assuming no leaks, then look at Suburban operations manual linked to below (You did not say if you have a Suburban or Atwood water heater, but from your description it sounds like it could be the Suburban SW6DE (Suburban, Water Heater, 6 gl, Direct Spark, Electric)).
Water Heater - Suburban Operation
Page 22 has a troubleshooting guide that is helpful. This is the regular user manual for that model for your reference -- Water Heater - Suburban 6 Gallon User Guide
Both the gas has to run freely and the spark has to spark correctly and at the right time. When done correctly, there will be an audible pop as it lights, but not a boom. The boom indicates that either the gas or the spark or both are a problem. If the spark is not right or is not located correctly in the flow of gas then too much gas builds up and ignites with a boom. I guess this only happens with yours when the door is closed allowing the gas to build up faster.
These are things the dealer really should have checked for when they looked at it for you. But, if you want to do it yourself ...
Clean the burner tube. A common problem as mentioned by chevy02 is a partially blocked tube from a small spider web. I don't know why, but spiders love to make webs in these things. I have had this issue before and blowing out the tube or cleaning it some other way usually corrects the issue.
If that doesn't work, then try to adjust the spark gap between the two electrodes. The gap should be about 1/8" (see page 22). There are two probes so measure the gap. You can bend the non-ceramic carefully. Do not break the probes.
Also, its possible the probes are not aligned correctly within the flow of gas. Reposition the electrode assembly so that it is in the flow of gas. IIRC (because I am not in front of my camper
) you can loosen the electrode assembly mounting screw and shift the assembly slightly.
Let us know how it goes. Good luck! And, Welcome to the forum!!