The Walmart/MoneyGram “Pay Bills” FAQ details how a payer (a tenant) can pay a bill at Walmart or Kroger. It appears that the payee (you, the park owner) can set up an account with Walmart/Moneygram to receive funds.
My educated guess is that the local utilities don’t send a clerk down to the local Walmart every time someone pays a water bill; billers must set up an ACH arrangement with MoneyGram. There’s an 800 number on that FAQ page for contacting MoneyGram. It might be worth a call to see what’s involved with setting up an account, the fees, and such.
One further thought — why not simply have tenants send their payments to your physical address?