WOW...
A 1975
Boler that was made in Earlton Ontario just two weeks ago??? What a find LOLOLOL...
Most/Many/A lot of/some? service doors use a standard CH751 key that you can get at almost any key shop or RV dealer. Also available all over on-line. Common keys like this are the bane of the business, but it's been that way for more than a few years..
Just a WAG... If you can't see inside the door from under a bench or cabinet, it may be a
battery compartment.