You get what you get in mileage. I have never checked mine. Towing with a big truck with a big motor and a non aerodynamic box/wind sail behind mileage is not something to be concerned about. You get what you get.
I have a 25 gallon tank that I purchased from TSC farm supply that fits in the back of my pickup. I installed a simple home garden spigot. Fill the tank with water, hook up the water hose to the tank and connect to my trailer water tank filler and turn on the valve and let gravity to the rest. Works fine. The empty 25 gallon tank is light and stores in the trailer when the trailer is in storage.
I take my F-250 in once per year to a place called Touch up in a Flash. They take out all the scratches and nicks and paint where necessary and I get back a showroom truck, totally detailed for around $350. They do a perfect match-up.
Dunedin RV Resort is excellent and across the road from the Pinellas Trail. We pretty much biked to everything. Publix and CVS nearby right on the trail and many restaurants and shopping within easy biking distance. Often biked out to Honeymoon Island Beach and into Ozona, Palm Harbour and Tarpon Springs. RV Park is excellent, well maintained and great facilities. Nice fresh fruit and vegetable stand accross the road at the access to the Pinellas trail. If you stay there make sure you go to Leukens Liquor Store in Dunedin. Huge selection and the best prices we have seen.