I think it depends on WHERE you are, WHAT you are offering, and the particular park.
Some of us full-timers need things when we are at a park (even if it a park where people only visit for short periods of time).
Fix up a great folder showing what you offer or flyers. Have business cards -- personally get out to all the campgrounds in your area to introduce yourself etc. Then follow up with emails.
This is very common in many of the parks we go to -- propane dealers deliver, mobile washes, mobile pet groomers, mobile repair etc. Perhaps contact them to find out about how they are insured, receive pmt.
We provide services and take cash, check, and PayPal. My main website with a PayPal button for payment (no charge). Products are ordered thru different site that takes credit cards.
Hope this helps.