Facility Expansion, RV's move out

First off welcome to the forum... Unfortunately I do not have a premade letter to share. I have done it in the past for the exact same reason at a facility. Assuming your state doesn't have any funny laws to deal with it's a matter of sending the letter *typically 30 day notice requirement*, state that they have to be vacated by a set date, if not vacated you will have no choice but to have the vehicle towed at the owners *their* expense. Let them know while you appreciate their business unfortunately parking spaces will no longer be offered at the facility. 

Make sure anyone coming in trying to make payments on any parking spots are dealt with then and future payments are not taken. Also make sure the date the letter states as a final date is WELL before the actual time you plan to do construction as you will always have holdovers that don't get things done in time. Personally I would find 1-2 other facilities in the area that do allow parking that I consider to be decent enough to recommend to the customers and list those options in the letter. This helps them find a new place and it helps you build some good connections with competitors if you don't have them already.