Can't Post Form to WordPress Page

I am posting a custom form on a WordPress page to another WordPress page. Apparently there are problem in doing this due to apparently how WordPress services pages using post variables.

I changed all of the fields of my first, short form so they would not be the same as any WordPress variables. That didn’t fix the problem of the short form post variables not making it to the 2nd page with the long form. When the short form at:
https://royalrestrooms.com/portable-restroom-reservations-royal-restrooms-4/
is posted to this page with another bigger form:
https://royalrestrooms.com/reservation-details/
the data from the short form doesn’t post to the 2nd long form. The email I get from that post has all blank fields.

These pages talk about the problem:
https://www.cabeeb.com/2009/09/posting-a-form-to-a-wordpress-page/
https://www.ads-software.com/support/topic/submitting-form-to-wordpress-page
https://www.ads-software.com/support/topic/passing-php-variable-cuases-error-404

When the short form posts to the big form page correctly the following will happen:
1. In the short form at https://royalrestrooms.com/portable-restroom-reservations-royal-restrooms-4/ when you select Type of Rental = Restroom Trailer Delivered you’ll see this on the next page:
2. The first title bar saying Restroom Trailer Reservation Information instead of Reservation Information.
3. Under the headings “Unit Type # People Capacity # Units Desired” you’ll see other rows of data and textboxes to enter numbers, but they’re not there.

When the files that are included into the top 2 files in WordPress were tested as stand-alone pages and the posts tested they worked find. The problem is that WordPress uses form variables in their delivery of pages that conflicts with the fields in these forms. The problem is posting a form to a WordPress page.

They are included in the top 2 pages above using the plugin Inline PHP to execute the PHP include code.

Any ideas?
Thanks for the help in advance…