Pre-fill Ontario Parks reservation form to speed through reservation attempts. To customize for your situation, edit variables in "User Settings" block.

OntarioParksReservation-1-Home.user.js

// ==UserScript==
// @name         OntarioParksReservation-1-Home
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  Pre-fill Ontario Parks reservation form
// @author       Mogsdad, David Bingham
// @match        https://reservations.ontarioparks.com/Home.aspx
// @match        https://reservations.ontarioparks.com/BrowseMaps
// @match        https://reservations.ontarioparks.com/OntarioParks%2fSoutheastParks%3fMap
// @match        https://reservations.ontarioparks.com/Sandbanks%2fOutletRiverAandB%2fOutletRiverA%3fMap
// @match        https://reservations.ontarioparks.com/Sandbanks/OutletRiverAandB/OutletRiverA?Map
// @match        https://reservations.ontarioparks.com/Sandbanks/OutletRiverAandB?Map
// @match        https://reservations.ontarioparks.com/Viewer.aspx
// @grant        none
// ==/UserScript==

(function() {
    'use strict';

    // Redirect to the campsite map
    let campsiteURL = "https://reservations.ontarioparks.com/Sandbanks/OutletRiverAandB/OutletRiverA?Map";
    if (window.location.href !== campsiteURL) {
        window.location = campsiteURL;
        return;
    }

    let $ = jQuery; // Avoid editor warnings; jQuery is provided in target site

    // Extend Date object to facilitate math-with-dates
    Date.prototype.addDays = function(days) {
        var date = new Date(this.valueOf());
        date.setDate(date.getDate() + days);
        return date;
    }
    Date.prototype.addMonths = function (m) {
        var d = new Date(this);
        var years = Math.floor(m / 12);
        var months = m - (years * 12);
        if (years) d.setFullYear(d.getFullYear() + years);
        if (months) d.setMonth(d.getMonth() + months);
        return d;
    }
    let today = new Date();
    today.setHours(0,0,0,0);

    // Settings for form pre-population; edit these for your situation.  <================== Settings
    let myResType = 'Campsite',
        myArrMth = new Date('June 2019'),
        myArrDay = today.addMonths( 5 ), // Arrive 5 months from today
        myNumNights = '23', // Max 23 nights
        myLocation = 'Sandbanks',
        myMap = 'Outlet River A and B',
        myCampground = 'Outlet River A',
        myEquipment = 'Trailer or RV up to 18ft',
        myPartySize = '4',
        mySite = '85';

    // Fill in reservation settings
    let myResTypeVal = findOption("selResType",myResType);
    if ($("#selResType").val() !== myResTypeVal) $("#selResType").val(myResTypeVal).change();
    setLatestQuickDate();
    let myArrMthFull = myArrMth.toString();
    if ($("#selArrMth").val() !== myArrMthFull) {
        $("#selArrMth").val(myArrMthFull).change();
    }
    let myArrDayFull = myArrDay.toString();
    if ($("#selArrDay").val() !== myArrDayFull) $("#selArrDay").val(myArrDayFull).change();
    if ($("#selNumNights").val() !== myNumNights) $("#selNumNights").val(myNumNights).change();

    let myLocationGUID = findOption("selLocation",myLocation) || findOption("MainContentPlaceHolder_LocationList",myLocation);
    if ( myLocationGUID && $("#selLocation,#MainContentPlaceHolder_LocationList").val() !== myLocationGUID ) {
        $("#selLocation,#MainContentPlaceHolder_LocationList").val(myLocationGUID).change();
    }
    let myMapGUID = findOption("selMap",myMap);
    if (myMapGUID && $("#selMap").val() !== myMapGUID) $("#selMap").val(myMapGUID);

    let myCampgroundGUID = findOption("MainContentPlaceHolder_MapList",myCampground);
    if (myCampgroundGUID && $("#MainContentPlaceHolder_MapList").val() !== myCampgroundGUID) $("#MainContentPlaceHolder_MapList").val(myCampgroundGUID);
    if ($("#selEquipmentSub").val() !== myEquipment) $("#selEquipmentSub").val(myEquipment);
    if ($("#selPartySize").val() !== myPartySize) $("#selPartySize").val(myPartySize);

    let mySiteGUID = findOption("selResource",mySite);
    if ( $("#selResource").val() !== mySiteGUID ) $("#selResource").val(mySiteGUID).change();

})();

/**
 * Return the option value corresponding to given optionText from given select element.
 *
 * @param {string} selList    Element ID of select list to search
 * @param {string} optionText HTML text to locate
 *
 * @returns {string}          Option value if match found, otherwise null.
 */
function findOption( selList, optionText ) {
    let selectList = selList && document.getElementById(selList);
    let options = selectList && selectList.options;
    if (! options ) {
        if (selectList) console.log('Failed to find ' + optionText + ' in ' + selList);
        return null;
    }

    var kvArray = Array.apply(null, options).map(function(el){return [el.text,el.value]});
    var kvMap = new Map(kvArray);

    return (kvMap.get(optionText));
}

/**
 * Set the QuickDateList that is latest in the year.
 */
function setLatestQuickDate() {
    let selectList = document.getElementById('MainContentPlaceHolder_QuickDateList');
    let options = selectList && selectList.options;
    if (! options ) {
        console.log('No QuickDateList found');
        return null;
    }

    var kvArray = Array.apply(null, options).map(function(el){return el.value});

    $("#MainContentPlaceHolder_QuickDateList").val(kvArray[kvArray.length-1]).change();
}