The water heater is not made by Jayco. It does seem odd that it was in a sealed area. You got a lemon. Sorry to hear that. Will the dealer take it back? What has Jayco said about a replacement unit? Have you called Jayco about the dimensions you are looking for. I have had good experiences when calling.
One bad experience does not make Jayco a bad brand. Every brand has these.
Our 2013 Jayfeather X20E is the 3rd Jayco trailer we've bought new. A 1208 pup in '93, owned for 10 years, a Kiwi 17a hybrid on '03, owned for 10 years, and on our 3rd season with the X20E. All had minor fit and finish issues but have been solid trailers otherwise.