I have imported two motorhomes into Canada, and cars too.
It isn't difficult or daunting.
The
Government of Canada info.
Pete's RV has good info.
You must notify US Customs at the exit port 72 hours in advance. They want to check for stolen vehicles. Stop at US Customs before crossing and present your docs.
You will pay GST at Cdn Customs. There is no duty. Complete the RIV (Registrar of Imported Vehicles) program, usually by inspection at Canadian Tire, but others can do it too. Provincial tax is paid when you register it in your province.
I doubt any customs broker will do all this for you. They mostly deal with duty and taxes. There may be agents that are familiar with the process but I don't know of any.