As Margi said, there are many things to do and see in Yuma. Lots of RV Parks with MANY amenities. Check out Fortuna D 'Oro, The Palms or Sundance on the west side of Foothill area. Or Westwind. 50 foot motorhome is very long???? What time of year are you coming. The place fills up January and February so a call ahead for a reservation would help secure your security of a place.
Yuma Territorial Prison, Old Town Main Street. Lots of off road trails. The lots in Yuma are the most serene and private but if you are only staying a week most of those rentals are for several months or more. Check out the Yuma Visitor website for daily activities. Of course Algodones, Mexico is good for a visit for medications, glasses, dental and plain old tourist shopping. Bring a passport and money for the parking lot on US side and then you walk over border. A really fun day!