If you trust the place you have it, let them look things over and advise you.
While doing that, you might ask Roger his input and as a sounding board to what they tell you...
RV Hydronic Heater Forum
Hope it is a simple problem, but if not, suggest you look at the 'upgraded parts replacements' for our era (we have a 2004 Allure High Cascade, with most likely the sam Hydro Hot unit). He commented to me on earlier posts on his board, about a few weak parts that should be considered for 'preventative maintenance'. (Which I have not done completely yet, but will the next time the unit is in for service...)
Best of luck to you,