$50K will certainly be enough given the parameters you've outlined above. Maintenance on a dsl that's used as you've outlined will set you back around $500/yr. This includes annual maintenance, oil change, lube, fuel filter changes etc. If you're concerned about turbo coolers, etc. an extended warranty will cost you in the neighborhood of $1k/yr and you're done.
We spend around $35-40/yr, travel extensively, 16 States last year, and stay in those private parks you referred to...:W....we do have RPI, TT and PA memberships to help offset the cost of the parks. Our stays average 7 days in any one spot. Biggest expense is food and fuel. Food you need whether you're in a stix/brix home or a motor home. Fuel has to simply be factored in.
We RV to enjoy life and see the country. We don't worry about our spending as our "habits" are pretty conservative. We simply enjoy life and pay the bills once a month as they come due. We might cut back one month if we splurged too much the previous month, but that's a rare occurrence. Enjoy the trip!....Dennis