I never declared any work done on the RV or on the truck in the USA, and I have done a few. Bottom line, for canadian, we have to delcare goods bought in USA, this would include parts bought in USA for a vehicle. I never seen that you have to declare work done in USA. How would they know anyway?
You are certainly allowed a certain amount of goods when returning to your country without paying duties. I do not know in which amount for US resident but for Canadians it is 800$ a person for over 48 hours. If you are within your allowance, just declare and forget about it.
If you are over the limits, for us Canadians, we only pay taxes, so it is not a big deal anyway.
One thing could happen if you do not declare and they search you wallet and find a bills of sale that you did not declare, or you show your bills of goodies you bought and in all the bills, there is this one you bought and did not declare. I did happen me once that they searched my wallet and asked questions about some bills. If they find it, just say honestly that you did not know you had to declare it. Normally it would never go as far, unless they suspect something.