I am having issues with my 3 year old Dometic as well, it cools, just not very well and has been freezing up.
From my research the only cure for most of the scenarios is replacement of the unit, which is not setting too well for something that has been used so infrequently.
I reached out to Dometic, through the support link, on the their web site and have traded a couple of emails, but nothing in the way of "yea, we have been seeing problems with units from that time period and will do this...
You should reach out to them too.
I have read lots of posts, on various forums, from folks that had to have their units replaced under warranty, some more than once.
Good luck, keep us posted.
2014 Bullet Premier 22RBPR - let the camping commence!
2013 F150 Platinum - 5.0 - 3.55 ELD + towing package