A max of $3-400 a month could complicate things. Wisconsin and upper Michigan is basically a vacation mecca during the summer.
Pull up RVPARKREVIEWS.com and pick out some of the towns in northern Wisconsin or northern Minnesota. Then call a few of the parks near them and ask about monthly rates.
I've wandered Wisc and the Mich UP the last 2 years and never had a problem finding a good place to stay for 1 or 2 weeks. But maybe not at the rates you're looking for.
One thing - I can tell you that some of the owners are snowbirds too and are in Texas/Florida/Arizona right now, so you may not get an answer.